- 补充 当初始化 ConnectionString 参数为空时，给出友好错误提示；

FreeSql.Tests/UnitTest1.cs

@@ -123,9 +123,18 @@ namespace FreeSql.Tests {
 			}
 		}
 
+		public class TestEnum {
+			public Guid id { get; set; }
+			public Enum em { get; set; }
+		}
+
 		[Fact]
 		public void Test1() {
 
+			
+			g.sqlite.Insert(new TestEnum { }).ExecuteAffrows();
+			var telist = g.sqlite.Select<TestEnum>().ToList();
+
 			Assert.Throws<Exception>(() => g.sqlite.CodeFirst.SyncStructure<TestEnumable>());
 
 			var TestEnumable = new TestEnumable();

FreeSql/FreeSqlBuilder.cs

@@ -103,6 +103,7 @@ namespace FreeSql {
 
 		public IFreeSql Build() => Build<IFreeSql>();
 		public IFreeSql<TMark> Build<TMark>() {
+			if (string.IsNullOrEmpty(_masterConnectionString)) throw new Exception("参数 masterConnectionString 不可为空，请检查 UseConnectionString");
 			IFreeSql<TMark> ret = null;
 			Type type = null;
 			switch(_dataType) {