Translate non-aggregate string.Join to CONCAT_WS on SQL Server

src/EFCore.SqlServer/Query/Internal/SqlServerSqlTranslatingExpressionVisitor.cs

@@ -5,6 +5,7 @@
 using System.Globalization;
 using System.Text;
 using Microsoft.EntityFrameworkCore.Query.SqlExpressions;
+using Microsoft.EntityFrameworkCore.SqlServer.Infrastructure.Internal;
 using ExpressionExtensions = Microsoft.EntityFrameworkCore.Query.ExpressionExtensions;
 
 namespace Microsoft.EntityFrameworkCore.SqlServer.Query.Internal;
@@ -19,6 +20,8 @@ public class SqlServerSqlTranslatingExpressionVisitor : RelationalSqlTranslating
 {
     private readonly SqlServerQueryCompilationContext _queryCompilationContext;
     private readonly ISqlExpressionFactory _sqlExpressionFactory;
+    private readonly IRelationalTypeMappingSource _typeMappingSource;
+    private readonly int _sqlServerCompatibilityLevel;
 
     private static readonly HashSet<string> DateTimeDataTypes
         =
@@ -59,6 +62,9 @@ private static readonly MethodInfo StringEndsWithMethodInfo
     private static readonly MethodInfo StringContainsMethodInfo
         = typeof(string).GetRuntimeMethod(nameof(string.Contains), [typeof(string)])!;
 
+    private static readonly MethodInfo StringJoinMethodInfo
+        = typeof(string).GetRuntimeMethod(nameof(string.Join), [typeof(string), typeof(string[])])!;
+
     private static readonly MethodInfo EscapeLikePatternParameterMethod =
         typeof(SqlServerSqlTranslatingExpressionVisitor).GetTypeInfo().GetDeclaredMethod(nameof(ConstructLikePatternParameter))!;
 
@@ -74,11 +80,14 @@ private static readonly MethodInfo StringContainsMethodInfo
     public SqlServerSqlTranslatingExpressionVisitor(
         RelationalSqlTranslatingExpressionVisitorDependencies dependencies,
         SqlServerQueryCompilationContext queryCompilationContext,
-        QueryableMethodTranslatingExpressionVisitor queryableMethodTranslatingExpressionVisitor)
+        QueryableMethodTranslatingExpressionVisitor queryableMethodTranslatingExpressionVisitor,
+        ISqlServerSingletonOptions sqlServerSingletonOptions)
         : base(dependencies, queryCompilationContext, queryableMethodTranslatingExpressionVisitor)
     {
         _queryCompilationContext = queryCompilationContext;
         _sqlExpressionFactory = dependencies.SqlExpressionFactory;
+        _typeMappingSource = dependencies.TypeMappingSource;
+        _sqlServerCompatibilityLevel = sqlServerSingletonOptions.CompatibilityLevel;
     }
 
     /// <summary>
@@ -199,6 +208,60 @@ protected override Expression VisitMethodCall(MethodCallExpression methodCallExp
             return translation3;
         }
 
+        // Translate non-aggregate string.Join to CONCAT_WS (for aggregate string.Join, see SqlServerStringAggregateMethodTranslator)
+        if (method == StringJoinMethodInfo
+            && methodCallExpression.Arguments[1] is NewArrayExpression newArrayExpression
+            && _sqlServerCompatibilityLevel >= 140)
+        {
+            if (TranslationFailed(methodCallExpression.Arguments[0], Visit(methodCallExpression.Arguments[0]), out var delimiter))
+            {
+                return QueryCompilationContext.NotTranslatedExpression;
+            }
+
+            var arguments = new SqlExpression[newArrayExpression.Expressions.Count + 1];
+            arguments[0] = delimiter!;
+
+            var isUnicode = delimiter!.TypeMapping?.IsUnicode == true;
+
+            for (var i = 0; i < newArrayExpression.Expressions.Count; i++)
+            {
+                var argument = newArrayExpression.Expressions[i];
+                if (TranslationFailed(argument, Visit(argument), out var sqlArgument))
+                {
+                    return QueryCompilationContext.NotTranslatedExpression;
+                }
+
+                // CONCAT_WS returns a type with a length that varies based on actual inputs (i.e. the sum of all argument lengths, plus
+                // the length needed for the delimiters). We don't know column values (or even parameter values, so we always return max.
+                // We do vary return varchar(max) or nvarchar(max) based on whether we saw any nvarchar mapping.
+                if (sqlArgument!.TypeMapping?.IsUnicode == true)
+                {
+                    isUnicode = true;
+                }
+
+                // CONCAT_WS filters out nulls, but string.Join treats them as empty strings; so coalesce (which is a no-op for non-nullable
+                // arguments).
+                arguments[i + 1] = sqlArgument switch
+                {
+                    ColumnExpression { IsNullable: false } => sqlArgument,
+                    SqlConstantExpression constantExpression => constantExpression.Value is null
+                        ? _sqlExpressionFactory.Constant(string.Empty)
+                        : constantExpression,
+                    _ => Dependencies.SqlExpressionFactory.Coalesce(sqlArgument, _sqlExpressionFactory.Constant(string.Empty))
+                };
+            }
+
+            // CONCAT_WS never returns null; a null delimiter is interpreted as an empty string, and null arguments are skipped
+            // (but we coalesce them above in any case).
+            return Dependencies.SqlExpressionFactory.Function(
+                "CONCAT_WS",
+                arguments,
+                nullable: false,
+                argumentsPropagateNullability: new bool[arguments.Length],
+                typeof(string),
+                _typeMappingSource.FindMapping(isUnicode ? "nvarchar(max)" : "varchar(max)"));
+        }
+
         return base.VisitMethodCall(methodCallExpression);
 
         bool TryTranslateStartsEndsWithContains(
@@ -515,6 +578,20 @@ private Expression TranslateByteArrayElementAccess(Expression array, Expression
                 : QueryCompilationContext.NotTranslatedExpression;
     }
 
+    [DebuggerStepThrough]
+    private static bool TranslationFailed(Expression? original, Expression? translation, out SqlExpression? castTranslation)
+    {
+        if (original != null
+            && translation is not SqlExpression)
+        {
+            castTranslation = null;
+            return true;
+        }
+
+        castTranslation = translation as SqlExpression;
+        return false;
+    }
+
     private static string? GetProviderType(SqlExpression expression)
         => expression.TypeMapping?.StoreType;
 }

src/EFCore.SqlServer/Query/Internal/SqlServerSqlTranslatingExpressionVisitorFactory.cs

@@ -1,6 +1,8 @@
 // Licensed to the .NET Foundation under one or more agreements.
 // The .NET Foundation licenses this file to you under the MIT license.
 
+using Microsoft.EntityFrameworkCore.SqlServer.Infrastructure.Internal;
+
 namespace Microsoft.EntityFrameworkCore.SqlServer.Query.Internal;
 
 /// <summary>
@@ -11,16 +13,20 @@ namespace Microsoft.EntityFrameworkCore.SqlServer.Query.Internal;
 /// </summary>
 public class SqlServerSqlTranslatingExpressionVisitorFactory : IRelationalSqlTranslatingExpressionVisitorFactory
 {
+    private readonly ISqlServerSingletonOptions _sqlServerSingletonOptions;
+
     /// <summary>
     ///     This is an internal API that supports the Entity Framework Core infrastructure and not subject to
     ///     the same compatibility standards as public APIs. It may be changed or removed without notice in
     ///     any release. You should only use it directly in your code with extreme caution and knowing that
     ///     doing so can result in application failures when updating to a new Entity Framework Core release.
     /// </summary>
     public SqlServerSqlTranslatingExpressionVisitorFactory(
-        RelationalSqlTranslatingExpressionVisitorDependencies dependencies)
+        RelationalSqlTranslatingExpressionVisitorDependencies dependencies,
+        ISqlServerSingletonOptions sqlServerSingletonOptions)
     {
         Dependencies = dependencies;
+        _sqlServerSingletonOptions = sqlServerSingletonOptions;
     }
 
     /// <summary>
@@ -40,5 +46,6 @@ public virtual RelationalSqlTranslatingExpressionVisitor Create(
         => new SqlServerSqlTranslatingExpressionVisitor(
             Dependencies,
             (SqlServerQueryCompilationContext)queryCompilationContext,
-            queryableMethodTranslatingExpressionVisitor);
+            queryableMethodTranslatingExpressionVisitor,
+            _sqlServerSingletonOptions);
 }

test/EFCore.Cosmos.FunctionalTests/Query/NorthwindFunctionsQueryCosmosTest.cs

@@ -1940,6 +1940,9 @@ public override Task String_Join_with_ordering(bool async)
     public override Task String_Join_over_nullable_column(bool async)
         => AssertTranslationFailed(() => base.String_Join_over_nullable_column(async));
 
+    public override Task String_Join_non_aggregate(bool async)
+        => AssertTranslationFailed(() => base.String_Join_non_aggregate(async));
+
     public override Task String_Concat(bool async)
         => AssertTranslationFailed(() => base.String_Concat(async));
 

test/EFCore.Specification.Tests/Query/NorthwindFunctionsQueryTestBase.cs

@@ -271,6 +271,17 @@ public virtual Task String_Join_over_nullable_column(bool async)
                     a.Regions.Split("|").OrderBy(id => id).ToArray());
             });
 
+    [ConditionalTheory]
+    [MemberData(nameof(IsAsyncData))]
+    public virtual Task String_Join_non_aggregate(bool async)
+    {
+        var foo = "foo";
+
+        return AssertQuery(
+            async,
+            ss => ss.Set<Customer>().Where(c => string.Join("|", new[] { c.CompanyName, foo, null, "bar" }) == "Around the Horn|foo||bar"));
+    }
+
     [ConditionalTheory]
     [MemberData(nameof(IsAsyncData))]
     public virtual Task String_Concat(bool async)

test/EFCore.SqlServer.FunctionalTests/Query/NorthwindFunctionsQuerySqlServerTest.cs

@@ -359,6 +359,21 @@ GROUP BY [c].[City]
 """);
     }
 
+    [SqlServerCondition(SqlServerCondition.SupportsFunctions2017)]
+    public override async Task String_Join_non_aggregate(bool async)
+    {
+        await base.String_Join_non_aggregate(async);
+
+        AssertSql(
+            """
+@__foo_0='foo' (Size = 4000)
+
+SELECT [c].[CustomerID], [c].[Address], [c].[City], [c].[CompanyName], [c].[ContactName], [c].[ContactTitle], [c].[Country], [c].[Fax], [c].[Phone], [c].[PostalCode], [c].[Region]
+FROM [Customers] AS [c]
+WHERE CONCAT_WS(N'|', [c].[CompanyName], COALESCE(@__foo_0, N''), N'', N'bar') = N'Around the Horn|foo||bar'
+""");
+    }
+
     [SqlServerCondition(SqlServerCondition.SupportsFunctions2017)]
     public override async Task String_Concat(bool async)
     {

test/EFCore.Sqlite.FunctionalTests/Query/NorthwindFunctionsQuerySqliteTest.cs

@@ -989,6 +989,9 @@ GROUP BY "c"."City"
 """);
     }
 
+    public override Task String_Join_non_aggregate(bool async)
+        => AssertTranslationFailed(() => base.String_Join_non_aggregate(async));
+
     public override async Task String_Concat(bool async)
     {
         await base.String_Concat(async);