String literals handling.

Author: cincuranet

Provider/src/FirebirdSql.EntityFrameworkCore.Firebird.Tests/Migrations/MigrationsTests.cs

@@ -122,8 +122,8 @@ public void CreateTable()
     ""Id_Sequence"" INTEGER NOT NULL,
     ""EmployerId"" INTEGER,
     ""SSN"" char(11),
-    ""DEF_O"" VARCHAR(20) DEFAULT CAST(_UTF8'test' as VARCHAR(4) CHARACTER SET UTF8),
-    ""DEF_S"" VARCHAR(20) DEFAULT ('x'),
+    ""DEF_O"" VARCHAR(20) DEFAULT _UTF8'test',
+    ""DEF_S"" VARCHAR(20) DEFAULT 'x',
     PRIMARY KEY (""Id""),
     UNIQUE (""SSN""),
     FOREIGN KEY (""EmployerId"") REFERENCES ""Companies"" (""Id"") ON UPDATE NO ACTION ON DELETE NO ACTION

Provider/src/FirebirdSql.EntityFrameworkCore.Firebird/Query/Sql/Internal/FbQuerySqlGenerator.cs

@@ -13,7 +13,7 @@
  *    All Rights Reserved.
  */
 
-//$Authors = Jiri Cincura (jiri@cincura.net), Jean Ressouche, Rafael Almeida (ralms@ralms.net)
+//$Authors = Jiri Cincura (jiri@cincura.net)
 
 using System.Linq;
 using System.Linq.Expressions;
@@ -170,6 +170,24 @@ protected override string GenerateOperator(Expression expression)
 			return base.GenerateOperator(expression);
 		}
 
+		protected override Expression VisitConstant(ConstantExpression constantExpression)
+		{
+			var svalue = constantExpression.Value as string;
+			var isVarcharHack = constantExpression.Type == typeof(string) && svalue?.Length > 0;
+			if (isVarcharHack)
+			{
+				Sql.Append("CAST(");
+			}
+			base.VisitConstant(constantExpression);
+			if (isVarcharHack)
+			{
+				Sql.Append(" AS VARCHAR(");
+				Sql.Append(svalue.Length);
+				Sql.Append(") CHARACTER SET UTF8)");
+			}
+			return constantExpression;
+		}
+
 		public virtual Expression VisitSubstring(FbSubstringExpression substringExpression)
 		{
 			Sql.Append("SUBSTRING(");

Provider/src/FirebirdSql.EntityFrameworkCore.Firebird/Storage/Internal/FbStringTypeMapping.cs

@@ -13,7 +13,7 @@
  *    All Rights Reserved.
  */
 
-//$Authors = Jiri Cincura (jiri@cincura.net), Jean Ressouche, Rafael Almeida (ralms@ralms.net)
+//$Authors = Jiri Cincura (jiri@cincura.net)
 
 using System.Data.Common;
 using FirebirdSql.Data.FirebirdClient;
@@ -39,11 +39,9 @@ protected override void ConfigureParameter(DbParameter parameter)
 		protected override string GenerateNonNullSqlLiteral(object value)
 		{
 			var svalue = (string)value;
-			if (svalue == string.Empty)
-				return "''";
 			return IsUnicode
-				? $"CAST(_UTF8'{EscapeSqlLiteral(svalue)}' as VARCHAR({svalue.Length}) CHARACTER SET UTF8)"
-				: $"CAST('{EscapeSqlLiteral(svalue)}' as VARCHAR({svalue.Length}))";
+				? $"_UTF8'{EscapeSqlLiteral(svalue)}'"
+				: $"'{EscapeSqlLiteral(svalue)}'";
 		}
 	}
 }