- 增加 "x1".First/FirstOrDefault 表达式函数解析；

FreeSql.Tests/FreeSql.Tests.Provider.MySqlConnector/MySqlConnector/MySqlConnectorExpression/StringTest.cs

@@ -52,6 +52,19 @@ namespace FreeSql.Tests.MySqlConnectorExpression
             list.Add(g.mysql.Select<TestEqualsGuid>().Where(a => a.IsDeleted.Equals(false)).ToList());
         }
 
+        [Fact]
+        public void First()
+        {
+            Assert.Equal('x', select.First(a => "x1".First()));
+            Assert.Equal('z', select.First(a => "z1".First()));
+        }
+        [Fact]
+        public void FirstOrDefault()
+        {
+            Assert.Equal('x', select.First(a => "x1".FirstOrDefault()));
+            Assert.Equal('z', select.First(a => "z1".FirstOrDefault()));
+        }
+
         [Fact]
         public void Format()
         {

FreeSql.Tests/FreeSql.Tests.Provider.Odbc/Dameng/DamengExpression/StringTest.cs

@@ -50,6 +50,19 @@ namespace FreeSql.Tests.Odbc.DamengExpression
             list.Add(g.dameng.Select<TestEqualsGuid>().Where(a => a.id.Equals(Guid.Empty)).ToList());
         }
 
+        [Fact]
+        public void First()
+        {
+            Assert.Equal('x', select.First(a => "x1".First()));
+            Assert.Equal('z', select.First(a => "z1".First()));
+        }
+        [Fact]
+        public void FirstOrDefault()
+        {
+            Assert.Equal('x', select.First(a => "x1".FirstOrDefault()));
+            Assert.Equal('z', select.First(a => "z1".FirstOrDefault()));
+        }
+
         [Fact]
         public void Format()
         {

FreeSql.Tests/FreeSql.Tests.Provider.Odbc/Default/OdbcExpression/StringTest.cs

@@ -50,6 +50,19 @@ namespace FreeSql.Tests.Odbc.DefaultExpression
             list.Add(g.odbc.Select<TestEqualsGuid>().Where(a => a.id.Equals(Guid.Empty)).ToList());
         }
 
+        [Fact]
+        public void First()
+        {
+            Assert.Equal('x', select.First(a => "x1".First()));
+            Assert.Equal('z', select.First(a => "z1".First()));
+        }
+        [Fact]
+        public void FirstOrDefault()
+        {
+            Assert.Equal('x', select.First(a => "x1".FirstOrDefault()));
+            Assert.Equal('z', select.First(a => "z1".FirstOrDefault()));
+        }
+
         [Fact]
         public void Empty()
         {

FreeSql.Tests/FreeSql.Tests.Provider.Odbc/KingbaseES/KingbaseESExpression/StringTest.cs

@@ -50,6 +50,19 @@ namespace FreeSql.Tests.Odbc.KingbaseESExpression
             list.Add(g.kingbaseES.Select<TestEqualsGuid>().Where(a => a.id.Equals(Guid.Empty)).ToList());
         }
 
+        [Fact]
+        public void First()
+        {
+            Assert.Equal('x', select.First(a => "x1".First()));
+            Assert.Equal('z', select.First(a => "z1".First()));
+        }
+        [Fact]
+        public void FirstOrDefault()
+        {
+            Assert.Equal('x', select.First(a => "x1".FirstOrDefault()));
+            Assert.Equal('z', select.First(a => "z1".FirstOrDefault()));
+        }
+
         [Fact]
         public void Format()
         {

FreeSql.Tests/FreeSql.Tests.Provider.Odbc/MySql/MySqlExpression/StringTest.cs

@@ -52,6 +52,19 @@ namespace FreeSql.Tests.Odbc.MySqlExpression
             list.Add(g.mysql.Select<TestEqualsGuid>().Where(a => a.IsDeleted.Equals(false)).ToList());
         }
 
+        [Fact]
+        public void First()
+        {
+            Assert.Equal('x', select.First(a => "x1".First()));
+            Assert.Equal('z', select.First(a => "z1".First()));
+        }
+        [Fact]
+        public void FirstOrDefault()
+        {
+            Assert.Equal('x', select.First(a => "x1".FirstOrDefault()));
+            Assert.Equal('z', select.First(a => "z1".FirstOrDefault()));
+        }
+
         [Fact]
         public void Format()
         {

FreeSql.Tests/FreeSql.Tests.Provider.Odbc/Oracle/OracleExpression/StringTest.cs

@@ -50,6 +50,19 @@ namespace FreeSql.Tests.Odbc.OracleExpression
             list.Add(g.oracle.Select<TestEqualsGuid>().Where(a => a.id.Equals(Guid.Empty)).ToList());
         }
 
+        [Fact]
+        public void First()
+        {
+            Assert.Equal('x', select.First(a => "x1".First()));
+            Assert.Equal('z', select.First(a => "z1".First()));
+        }
+        [Fact]
+        public void FirstOrDefault()
+        {
+            Assert.Equal('x', select.First(a => "x1".FirstOrDefault()));
+            Assert.Equal('z', select.First(a => "z1".FirstOrDefault()));
+        }
+
         [Fact]
         public void Format()
         {

FreeSql.Tests/FreeSql.Tests.Provider.Odbc/PostgreSQL/PostgreSQLExpression/StringTest.cs

@@ -50,6 +50,19 @@ namespace FreeSql.Tests.Odbc.PostgreSQLExpression
             list.Add(g.pgsql.Select<TestEqualsGuid>().Where(a => a.id.Equals(Guid.Empty)).ToList());
         }
 
+        [Fact]
+        public void First()
+        {
+            Assert.Equal('x', select.First(a => "x1".First()));
+            Assert.Equal('z', select.First(a => "z1".First()));
+        }
+        [Fact]
+        public void FirstOrDefault()
+        {
+            Assert.Equal('x', select.First(a => "x1".FirstOrDefault()));
+            Assert.Equal('z', select.First(a => "z1".FirstOrDefault()));
+        }
+
         [Fact]
         public void Format()
         {

FreeSql.Tests/FreeSql.Tests.Provider.Odbc/SqlServer/SqlServerExpression/StringTest.cs

@@ -51,6 +51,19 @@ namespace FreeSql.Tests.Odbc.SqlServerExpression
             list.Add(g.sqlserver.Select<TestEqualsGuid>().Where(a => a.id.Equals(Guid.Empty)).ToList());
         }
 
+        [Fact]
+        public void First()
+        {
+            Assert.Equal('x', select.First(a => "x1".First()));
+            Assert.Equal('z', select.First(a => "z1".First()));
+        }
+        [Fact]
+        public void FirstOrDefault()
+        {
+            Assert.Equal('x', select.First(a => "x1".FirstOrDefault()));
+            Assert.Equal('z', select.First(a => "z1".FirstOrDefault()));
+        }
+
         [Fact]
         public void Format()
         {

FreeSql.Tests/FreeSql.Tests/Dameng/DamengExpression/StringTest.cs

@@ -50,6 +50,19 @@ namespace FreeSql.Tests.DamengExpression
             list.Add(g.dameng.Select<TestEqualsGuid>().Where(a => a.id.Equals(Guid.Empty)).ToList());
         }
 
+        [Fact]
+        public void First()
+        {
+            Assert.Equal('x', select.First(a => "x1".First()));
+            Assert.Equal('z', select.First(a => "z1".First()));
+        }
+        [Fact]
+        public void FirstOrDefault()
+        {
+            Assert.Equal('x', select.First(a => "x1".FirstOrDefault()));
+            Assert.Equal('z', select.First(a => "z1".FirstOrDefault()));
+        }
+
         [Fact]
         public void Format()
         {

FreeSql.Tests/FreeSql.Tests/DataContext/SqlServer/SqlServerFixture.cs

@@ -42,7 +42,7 @@ namespace FreeSql.Tests.DataContext.SqlServer
                 "TestTypeParentInfo23123", "xxdkdkdk1222", "xxx"};
             foreach (var tempTable in tempTables)
             {
-                DeleteTmpTable(dataTables, tempTable);
+                //DeleteTmpTable(dataTables, tempTable);
             }
         }
 

FreeSql.Tests/FreeSql.Tests/MsAccess/MsAccessExpression/StringTest.cs

@@ -50,6 +50,19 @@ namespace FreeSql.Tests.MsAccessExpression
             list.Add(g.msaccess.Select<TestEqualsGuid>().Where(a => a.id.Equals(Guid.Empty)).ToList());
         }
 
+        [Fact]
+        public void First()
+        {
+            Assert.Equal('x', select.First(a => "x1".First()));
+            Assert.Equal('z', select.First(a => "z1".First()));
+        }
+        [Fact]
+        public void FirstOrDefault()
+        {
+            Assert.Equal('x', select.First(a => "x1".FirstOrDefault()));
+            Assert.Equal('z', select.First(a => "z1".FirstOrDefault()));
+        }
+
         [Fact]
         public void Format()
         {

FreeSql.Tests/FreeSql.Tests/MySql/MySqlExpression/StringTest.cs

@@ -52,6 +52,19 @@ namespace FreeSql.Tests.MySqlExpression
             list.Add(g.mysql.Select<TestEqualsGuid>().Where(a => a.IsDeleted.Equals(false)).ToList());
         }
 
+        [Fact]
+        public void First()
+        {
+            Assert.Equal('x', select.First(a => "x1".First()));
+            Assert.Equal('z', select.First(a => "z1".First()));
+        }
+        [Fact]
+        public void FirstOrDefault()
+        {
+            Assert.Equal('x', select.First(a => "x1".FirstOrDefault()));
+            Assert.Equal('z', select.First(a => "z1".FirstOrDefault()));
+        }
+
         [Fact]
         public void Format()
         {

FreeSql.Tests/FreeSql.Tests/Oracle/OracleExpression/StringTest.cs

@@ -50,6 +50,19 @@ namespace FreeSql.Tests.OracleExpression
             list.Add(g.oracle.Select<TestEqualsGuid>().Where(a => a.id.Equals(Guid.Empty)).ToList());
         }
 
+        [Fact]
+        public void First()
+        {
+            Assert.Equal('x', select.First(a => "x1".First()));
+            Assert.Equal('z', select.First(a => "z1".First()));
+        }
+        [Fact]
+        public void FirstOrDefault()
+        {
+            Assert.Equal('x', select.First(a => "x1".FirstOrDefault()));
+            Assert.Equal('z', select.First(a => "z1".FirstOrDefault()));
+        }
+
         [Fact]
         public void Format()
         {

FreeSql.Tests/FreeSql.Tests/PostgreSQL/PostgreSQLExpression/StringTest.cs

@@ -50,6 +50,19 @@ namespace FreeSql.Tests.PostgreSQLExpression
             list.Add(g.pgsql.Select<TestEqualsGuid>().Where(a => a.id.Equals(Guid.Empty)).ToList());
         }
 
+        [Fact]
+        public void First()
+        {
+            Assert.Equal('x', select.First(a => "x1".First()));
+            Assert.Equal('z', select.First(a => "z1".First()));
+        }
+        [Fact]
+        public void FirstOrDefault()
+        {
+            Assert.Equal('x', select.First(a => "x1".FirstOrDefault()));
+            Assert.Equal('z', select.First(a => "z1".FirstOrDefault()));
+        }
+
         [Fact]
         public void Format()
         {

FreeSql.Tests/FreeSql.Tests/ShenTong/ShenTongExpression/StringTest.cs

@@ -50,6 +50,19 @@ namespace FreeSql.Tests.ShenTongExpression
             list.Add(g.shentong.Select<TestEqualsGuid>().Where(a => a.id.Equals(Guid.Empty)).ToList());
         }
 
+        [Fact]
+        public void First()
+        {
+            Assert.Equal('x', select.First(a => "x1".First()));
+            Assert.Equal('z', select.First(a => "z1".First()));
+        }
+        [Fact]
+        public void FirstOrDefault()
+        {
+            Assert.Equal('x', select.First(a => "x1".FirstOrDefault()));
+            Assert.Equal('z', select.First(a => "z1".FirstOrDefault()));
+        }
+
         [Fact]
         public void Format()
         {

FreeSql.Tests/FreeSql.Tests/SqlServer/SqlServerExpression/StringTest.cs

@@ -1,5 +1,6 @@
 using FreeSql.DataAnnotations;
 using FreeSql.Tests.DataContext.SqlServer;
+using Microsoft.EntityFrameworkCore.Internal;
 using System;
 using System.Collections.Generic;
 using System.Linq;
@@ -65,6 +66,19 @@ namespace FreeSql.Tests.SqlServerExpression
             list.Add(select.Where(a => a.TitleVarchar == "aaa").ToList());
         }
 
+        [Fact]
+        public void First()
+        {
+            Assert.Equal('x', select.First(a => "x1".First()));
+            Assert.Equal('z', select.First(a => "z1".First()));
+        }
+        [Fact]
+        public void FirstOrDefault()
+        {
+            Assert.Equal('x', select.First(a => "x1".FirstOrDefault()));
+            Assert.Equal('z', select.First(a => "z1".FirstOrDefault()));
+        }
+
         [Fact]
         public void Format()
         {

FreeSql.Tests/FreeSql.Tests/Sqlite/SqliteExpression/StringTest.cs

@@ -50,6 +50,19 @@ namespace FreeSql.Tests.SqliteExpression
             list.Add(g.sqlite.Select<TestEqualsGuid>().Where(a => a.id.Equals(Guid.Empty)).ToList());
         }
 
+        [Fact]
+        public void First()
+        {
+            Assert.Equal('x', select.First(a => "x1".First()));
+            Assert.Equal('z', select.First(a => "z1".First()));
+        }
+        [Fact]
+        public void FirstOrDefault()
+        {
+            Assert.Equal('x', select.First(a => "x1".FirstOrDefault()));
+            Assert.Equal('z', select.First(a => "z1".FirstOrDefault()));
+        }
+
         [Fact]
         public void Format()
         {

FreeSql/Internal/CommonExpression.cs

@@ -41,6 +41,7 @@ namespace FreeSql.Internal
                     field.Append(", ").Append(parent.DbField);
                     if (index >= 0) field.Append(_common.FieldAsAlias($"as{++index}"));
                     else if (index == ReadAnonymousFieldAsCsName && string.IsNullOrEmpty(parent.CsName) == false) field.Append(_common.FieldAsAlias(parent.CsName));
+                    if (parent.CsType == null) parent.CsType = exp.Type;
                     return false;
                 case ExpressionType.Convert: return ReadAnonymousField(_tables, field, parent, ref index, (exp as UnaryExpression)?.Operand, getSelectGroupingMapString, whereCascadeExpression, isAllDtoMap);
                 case ExpressionType.Constant:
@@ -60,6 +61,7 @@ namespace FreeSql.Internal
                     field.Append(", ").Append(parent.DbField);
                     if (index >= 0) field.Append(_common.FieldAsAlias($"as{++index}"));
                     else if (index == ReadAnonymousFieldAsCsName && string.IsNullOrEmpty(parent.CsName) == false) field.Append(_common.FieldAsAlias(parent.CsName));
+                    if (parent.CsType == null) parent.CsType = exp.Type;
                     return false;
                 case ExpressionType.Call:
                     var callExp = exp as MethodCallExpression;
@@ -77,6 +79,7 @@ namespace FreeSql.Internal
                     field.Append(", ").Append(parent.DbField);
                     if (index >= 0) field.Append(_common.FieldAsAlias($"as{++index}"));
                     else if (index == ReadAnonymousFieldAsCsName && string.IsNullOrEmpty(parent.CsName) == false) field.Append(_common.FieldAsAlias(parent.CsName));
+                    if (parent.CsType == null) parent.CsType = exp.Type;
                     return false;
                 case ExpressionType.Parameter:
                 case ExpressionType.MemberAccess:
@@ -262,6 +265,7 @@ namespace FreeSql.Internal
             field.Append(", ").Append(parent.DbField);
             if (index >= 0) field.Append(_common.FieldAsAlias($"as{++index}"));
             else if (index == ReadAnonymousFieldAsCsName && string.IsNullOrEmpty(parent.CsName) == false) field.Append(_common.FieldAsAlias(parent.CsName));
+            if (parent.CsType == null) parent.CsType = exp.Type;
             return false;
         }
         public object ReadAnonymous(ReadAnonymousTypeInfo parent, DbDataReader dr, ref int index, bool notRead, ReadAnonymousDbValueRef dbValue)

Providers/FreeSql.Provider.Dameng/DamengExpression.cs

@@ -100,6 +100,16 @@ namespace FreeSql.Dameng
                         objExp = callExp.Arguments.FirstOrDefault();
                         objType = objExp?.Type;
                         argIndex++;
+
+                        if (objType == typeof(string))
+                        {
+                            switch (callExp.Method.Name)
+                            {
+                                case "First":
+                                case "FirstOrDefault":
+                                    return $"substr({getExp(callExp.Arguments[0])}, 1, 1)";
+                            }
+                        }
                     }
                     if (objType == null) objType = callExp.Method.DeclaringType;
                     if (objType != null || objType.IsArrayOrList())

Providers/FreeSql.Provider.MsAccess/MsAccessExpression.cs

@@ -70,6 +70,16 @@ namespace FreeSql.MsAccess
                         objExp = callExp.Arguments.FirstOrDefault();
                         objType = objExp?.Type;
                         argIndex++;
+
+                        if (objType == typeof(string))
+                        {
+                            switch (callExp.Method.Name)
+                            {
+                                case "First":
+                                case "FirstOrDefault":
+                                    return $"left({getExp(callExp.Arguments[0])}, 1)";
+                            }
+                        }
                     }
                     if (objType == null) objType = callExp.Method.DeclaringType;
                     if (objType != null || objType.IsArrayOrList())

Providers/FreeSql.Provider.MySql/MySqlExpression.cs

@@ -98,6 +98,16 @@ namespace FreeSql.MySql
                         objExp = callExp.Arguments.FirstOrDefault();
                         objType = objExp?.Type;
                         argIndex++;
+
+                        if (objType == typeof(string))
+                        {
+                            switch (callExp.Method.Name)
+                            {
+                                case "First":
+                                case "FirstOrDefault":
+                                    return $"substr({getExp(callExp.Arguments[0])}, 1, 1)";
+                            }
+                        }
                     }
                     if (objType == null) objType = callExp.Method.DeclaringType;
                     if (objType != null || objType.IsArrayOrList())

Providers/FreeSql.Provider.Odbc/Dameng/OdbcDamengExpression.cs

@@ -100,6 +100,16 @@ namespace FreeSql.Odbc.Dameng
                         objExp = callExp.Arguments.FirstOrDefault();
                         objType = objExp?.Type;
                         argIndex++;
+
+                        if (objType == typeof(string))
+                        {
+                            switch (callExp.Method.Name)
+                            {
+                                case "First":
+                                case "FirstOrDefault":
+                                    return $"substr({getExp(callExp.Arguments[0])}, 1, 1)";
+                            }
+                        }
                     }
                     if (objType == null) objType = callExp.Method.DeclaringType;
                     if (objType != null || objType.IsArrayOrList())

Providers/FreeSql.Provider.Odbc/Default/OdbcExpression.cs

@@ -104,6 +104,16 @@ namespace FreeSql.Odbc.Default
                         objExp = callExp.Arguments.FirstOrDefault();
                         objType = objExp?.Type;
                         argIndex++;
+
+                        if (objType == typeof(string))
+                        {
+                            switch (callExp.Method.Name)
+                            {
+                                case "First":
+                                case "FirstOrDefault":
+                                    return _utils.Adapter.LambdaString_Substring(getExp(callExp.Arguments[0]), "1", "1");
+                            }
+                        }
                     }
                     if (objType == null) objType = callExp.Method.DeclaringType;
                     if (objType != null || objType.IsArrayOrList())

Providers/FreeSql.Provider.Odbc/KingbaseES/OdbcKingbaseESExpression.cs

@@ -101,6 +101,16 @@ namespace FreeSql.Odbc.KingbaseES
                         objExp = callExp.Arguments.FirstOrDefault();
                         objType = objExp?.Type;
                         argIndex++;
+
+                        if (objType == typeof(string))
+                        {
+                            switch (callExp.Method.Name)
+                            {
+                                case "First":
+                                case "FirstOrDefault":
+                                    return $"substr({getExp(callExp.Arguments[0])}, 1, 1)";
+                            }
+                        }
                     }
                     if (objType == null) objType = callExp.Method.DeclaringType;
                     if (objType != null || objType.IsArrayOrList())

Providers/FreeSql.Provider.Odbc/MySql/OdbcMySqlExpression.cs

@@ -96,6 +96,16 @@ namespace FreeSql.Odbc.MySql
                         objExp = callExp.Arguments.FirstOrDefault();
                         objType = objExp?.Type;
                         argIndex++;
+
+                        if (objType == typeof(string))
+                        {
+                            switch (callExp.Method.Name)
+                            {
+                                case "First":
+                                case "FirstOrDefault":
+                                    return $"substr({getExp(callExp.Arguments[0])}, 1, 1)";
+                            }
+                        }
                     }
                     if (objType == null) objType = callExp.Method.DeclaringType;
                     if (objType != null || objType.IsArrayOrList())

Providers/FreeSql.Provider.Odbc/Oracle/OdbcOracleExpression.cs

@@ -100,6 +100,16 @@ namespace FreeSql.Odbc.Oracle
                         objExp = callExp.Arguments.FirstOrDefault();
                         objType = objExp?.Type;
                         argIndex++;
+
+                        if (objType == typeof(string))
+                        {
+                            switch (callExp.Method.Name)
+                            {
+                                case "First":
+                                case "FirstOrDefault":
+                                    return $"substr({getExp(callExp.Arguments[0])}, 1, 1)";
+                            }
+                        }
                     }
                     if (objType == null) objType = callExp.Method.DeclaringType;
                     if (objType != null || objType.IsArrayOrList())

Providers/FreeSql.Provider.Odbc/PostgreSQL/OdbcPostgreSQLExpression.cs

@@ -101,6 +101,16 @@ namespace FreeSql.Odbc.PostgreSQL
                         objExp = callExp.Arguments.FirstOrDefault();
                         objType = objExp?.Type;
                         argIndex++;
+
+                        if (objType == typeof(string))
+                        {
+                            switch (callExp.Method.Name)
+                            {
+                                case "First":
+                                case "FirstOrDefault":
+                                    return $"substr({getExp(callExp.Arguments[0])}, 1, 1)";
+                            }
+                        }
                     }
                     if (objType == null) objType = callExp.Method.DeclaringType;
                     if (objType != null || objType.IsArrayOrList())

Providers/FreeSql.Provider.Odbc/SqlServer/OdbcSqlServerExpression.cs

@@ -106,6 +106,16 @@ namespace FreeSql.Odbc.SqlServer
                         objExp = callExp.Arguments.FirstOrDefault();
                         objType = objExp?.Type;
                         argIndex++;
+
+                        if (objType == typeof(string))
+                        {
+                            switch (callExp.Method.Name)
+                            {
+                                case "First":
+                                case "FirstOrDefault":
+                                    return $"substr({getExp(callExp.Arguments[0])}, 1, 1)";
+                            }
+                        }
                     }
                     if (objType == null) objType = callExp.Method.DeclaringType;
                     if (objType != null || objType.IsArrayOrList())

Providers/FreeSql.Provider.Oracle/OracleExpression.cs

@@ -100,6 +100,16 @@ namespace FreeSql.Oracle
                         objExp = callExp.Arguments.FirstOrDefault();
                         objType = objExp?.Type;
                         argIndex++;
+
+                        if (objType == typeof(string))
+                        {
+                            switch (callExp.Method.Name)
+                            {
+                                case "First":
+                                case "FirstOrDefault":
+                                    return $"substr({getExp(callExp.Arguments[0])}, 1, 1)";
+                            }
+                        }
                     }
                     if (objType == null) objType = callExp.Method.DeclaringType;
                     if (objType != null || objType.IsArrayOrList())

Providers/FreeSql.Provider.PostgreSQL/PostgreSQLExpression.cs

@@ -102,6 +102,16 @@ namespace FreeSql.PostgreSQL
                         objExp = callExp.Arguments.FirstOrDefault();
                         objType = objExp?.Type;
                         argIndex++;
+
+                        if (objType == typeof(string))
+                        {
+                            switch (callExp.Method.Name)
+                            {
+                                case "First":
+                                case "FirstOrDefault":
+                                    return $"substr({getExp(callExp.Arguments[0])}, 1, 1)";
+                            }
+                        }
                     }
                     if (objType == null) objType = callExp.Method.DeclaringType;
                     if (objType != null || objType.IsArrayOrList())

Providers/FreeSql.Provider.ShenTong/ShenTongExpression.cs

@@ -101,6 +101,16 @@ namespace FreeSql.ShenTong
                         objExp = callExp.Arguments.FirstOrDefault();
                         objType = objExp?.Type;
                         argIndex++;
+
+                        if (objType == typeof(string))
+                        {
+                            switch (callExp.Method.Name)
+                            {
+                                case "First":
+                                case "FirstOrDefault":
+                                    return $"substr({getExp(callExp.Arguments[0])}, 1, 1)";
+                            }
+                        }
                     }
                     if (objType == null) objType = callExp.Method.DeclaringType;
                     if (objType != null || objType.IsArrayOrList())

Providers/FreeSql.Provider.SqlServer/SqlServerExpression.cs

@@ -105,6 +105,16 @@ namespace FreeSql.SqlServer
                         objExp = callExp.Arguments.FirstOrDefault();
                         objType = objExp?.Type;
                         argIndex++;
+
+                        if (objType == typeof(string))
+                        {
+                            switch (callExp.Method.Name)
+                            {
+                                case "First":
+                                case "FirstOrDefault":
+                                    return $"substring({getExp(callExp.Arguments[0])}, 1, 1)";
+                            }
+                        }
                     }
                     if (objType == null) objType = callExp.Method.DeclaringType;
                     if (objType != null || objType.IsArrayOrList())

Providers/FreeSql.Provider.Sqlite/SqliteExpression.cs

@@ -96,6 +96,16 @@ namespace FreeSql.Sqlite
                         objExp = callExp.Arguments.FirstOrDefault();
                         objType = objExp?.Type;
                         argIndex++;
+
+                        if (objType == typeof(string))
+                        {
+                            switch (callExp.Method.Name)
+                            {
+                                case "First":
+                                case "FirstOrDefault":
+                                    return $"substr({getExp(callExp.Arguments[0])}, 1, 1)";
+                            }
+                        }
                     }
                     if (objType == null) objType = callExp.Method.DeclaringType;
                     if (objType != null || objType.IsArrayOrList())