Concat on string property Error

[GLuca74]

I am migrating a Test Project From EF Core 7.0.4 to 8.0.1
I have some tests that using 7.0.4 work, with 8.0.1 fail

the simplest query that reproduce is this :

db.Set<City>().Select(itm=>itm.CityName.Concat("- A string")).ToArray();


    public class City
    {
        public Guid CityID { get; set; }
        public string CityName { get; set; }
    }

the provider is SqlServer

the project at https://github.com/GLuca74/EFConcat reproduce the problem

the message is :

System.InvalidOperationException
  HResult=0x80131509
  Messaggio=The LINQ expression '[-, ,A, ,s,t,r,i,n,g]' could not be translated. Either rewrite the query in a form that can be translated, or switch to client evaluation explicitly by inserting a call to 'AsEnumerable', 'AsAsyncEnumerable', 'ToList', or 'ToListAsync'. See https://go.microsoft.com/fwlink/?linkid=2101038 for more information.
  Origine=Microsoft.EntityFrameworkCore.Relational
  Analisi dello stack:
   in Microsoft.EntityFrameworkCore.Query.Internal.RelationalProjectionBindingExpressionVisitor.VisitExtension(Expression extensionExpression)
   in Microsoft.EntityFrameworkCore.Query.Internal.RelationalProjectionBindingExpressionVisitor.Visit(Expression expression)
   in Microsoft.EntityFrameworkCore.Query.Internal.RelationalProjectionBindingExpressionVisitor.VisitMethodCall(MethodCallExpression methodCallExpression)
   in Microsoft.EntityFrameworkCore.Query.Internal.RelationalProjectionBindingExpressionVisitor.Visit(Expression expression)
   in Microsoft.EntityFrameworkCore.Query.Internal.RelationalProjectionBindingExpressionVisitor.Translate(SelectExpression selectExpression, Expression expression)
   in Microsoft.EntityFrameworkCore.Query.RelationalQueryableMethodTranslatingExpressionVisitor.TranslateSelect(ShapedQueryExpression source, LambdaExpression selector)
   in Microsoft.EntityFrameworkCore.Query.QueryableMethodTranslatingExpressionVisitor.VisitMethodCall(MethodCallExpression methodCallExpression)
   in Microsoft.EntityFrameworkCore.Query.RelationalQueryableMethodTranslatingExpressionVisitor.VisitMethodCall(MethodCallExpression methodCallExpression)
   in Microsoft.EntityFrameworkCore.Query.QueryableMethodTranslatingExpressionVisitor.Translate(Expression expression)
   in Microsoft.EntityFrameworkCore.Query.RelationalQueryableMethodTranslatingExpressionVisitor.Translate(Expression expression)
   in Microsoft.EntityFrameworkCore.Query.QueryCompilationContext.CreateQueryExecutor[TResult](Expression query)
   in Microsoft.EntityFrameworkCore.Storage.Database.CompileQuery[TResult](Expression query, Boolean async)
   in Microsoft.EntityFrameworkCore.Query.Internal.QueryCompiler.CompileQueryCore[TResult](IDatabase database, Expression query, IModel model, Boolean async)
   in Microsoft.EntityFrameworkCore.Query.Internal.QueryCompiler.<>c__DisplayClass9_0`1.<Execute>b__0()
   in Microsoft.EntityFrameworkCore.Query.Internal.CompiledQueryCache.GetOrAddQuery[TResult](Object cacheKey, Func`1 compiler)
   in Microsoft.EntityFrameworkCore.Query.Internal.QueryCompiler.Execute[TResult](Expression query)
   in Microsoft.EntityFrameworkCore.Query.Internal.EntityQueryProvider.Execute[TResult](Expression expression)
   in Microsoft.EntityFrameworkCore.Query.Internal.EntityQueryable`1.GetEnumerator()
   in System.Collections.Generic.LargeArrayBuilder`1.AddRange(IEnumerable`1 items)
   in System.Collections.Generic.EnumerableHelpers.ToArray[T](IEnumerable`1 source)
   in EFConcat.Program.Main(String[] args) in D:\VM\EFConcat\Program.cs: riga 17

Can you please check?
Thank you

[ajcvickers]

Note for team: still repros on latest daily; regression from 7.

Output from 8:

dbug: 2/12/2024 10:33:44.583 CoreEventId.QueryCompilationStarting[10111] (Microsoft.EntityFrameworkCore.Query) 
      Compiling query expression:
      'DbSet<City>()
          .Select(itm => itm.CityName
              .Concat("- A string"))'
Unhandled exception. System.InvalidOperationException: The LINQ expression '[-, ,A, ,s,t,r,i,n,g]' could not be translated. Either rewrite the query in a form that can be translated, or switch to client evaluation explicitly by inserting a call to 'AsEnumerable', 'AsAsyncEnumerable', 'ToList', or 'ToListAsync'. See https://go.microsoft.com/fwlink/?linkid=2101038 for more information.
   at Microsoft.EntityFrameworkCore.Query.Internal.RelationalProjectionBindingExpressionVisitor.VisitExtension(Expression extensionExpression)
   at Microsoft.EntityFrameworkCore.Query.Internal.RelationalProjectionBindingExpressionVisitor.Visit(Expression expression)
   at Microsoft.EntityFrameworkCore.Query.Internal.RelationalProjectionBindingExpressionVisitor.VisitMethodCall(MethodCallExpression methodCallExpression)
   at Microsoft.EntityFrameworkCore.Query.Internal.RelationalProjectionBindingExpressionVisitor.Visit(Expression expression)
   at Microsoft.EntityFrameworkCore.Query.Internal.RelationalProjectionBindingExpressionVisitor.Translate(SelectExpression selectExpression, Expression expression)
   at Microsoft.EntityFrameworkCore.Query.RelationalQueryableMethodTranslatingExpressionVisitor.TranslateSelect(ShapedQueryExpression source, LambdaExpression selector)
   at Microsoft.EntityFrameworkCore.Query.QueryableMethodTranslatingExpressionVisitor.VisitMethodCall(MethodCallExpression methodCallExpression)
   at Microsoft.EntityFrameworkCore.Query.RelationalQueryableMethodTranslatingExpressionVisitor.VisitMethodCall(MethodCallExpression methodCallExpression)
   at Microsoft.EntityFrameworkCore.Query.QueryableMethodTranslatingExpressionVisitor.Translate(Expression expression)
   at Microsoft.EntityFrameworkCore.Query.RelationalQueryableMethodTranslatingExpressionVisitor.Translate(Expression expression)
   at Microsoft.EntityFrameworkCore.Query.QueryCompilationContext.CreateQueryExecutor[TResult](Expression query)
   at Microsoft.EntityFrameworkCore.Storage.Database.CompileQuery[TResult](Expression query, Boolean async)
   at Microsoft.EntityFrameworkCore.Query.Internal.QueryCompiler.CompileQueryCore[TResult](IDatabase database, Expression query, IModel model, Boolean async)
   at Microsoft.EntityFrameworkCore.Query.Internal.QueryCompiler.<>c__DisplayClass9_0`1.<Execute>b__0()
   at Microsoft.EntityFrameworkCore.Query.Internal.CompiledQueryCache.GetOrAddQuery[TResult](Object cacheKey, Func`1 compiler)
   at Microsoft.EntityFrameworkCore.Query.Internal.QueryCompiler.Execute[TResult](Expression query)
   at Microsoft.EntityFrameworkCore.Query.Internal.EntityQueryProvider.Execute[TResult](Expression expression)
   at Microsoft.EntityFrameworkCore.Query.Internal.EntityQueryable`1.GetEnumerator()
   at System.Collections.Generic.LargeArrayBuilder`1.AddRange(IEnumerable`1 items)
   at System.Collections.Generic.EnumerableHelpers.ToArray[T](IEnumerable`1 source)
   at EFConcat.Program.Main(String[] args) in D:\code\repros\EFConcat-master\EFConcat-master\Program.cs:line 18

Output from 7:

dbug: 2/12/2024 10:32:10.245 CoreEventId.QueryCompilationStarting[10111] (Microsoft.EntityFrameworkCore.Query) 
      Compiling query expression:
      'DbSet<City>()
          .Select(itm => itm.CityName
              .Concat("- A string"))'
dbug: 2/12/2024 10:32:10.395 CoreEventId.QueryExecutionPlanned[10107] (Microsoft.EntityFrameworkCore.Query) 
      Generated query execution expression:
      'queryContext => new SingleQueryingEnumerable<IEnumerable<char>>(
          (RelationalQueryContext)queryContext,
          RelationalCommandCache.QueryExpression(
              Client Projections:
                  0 -> 0
              SELECT c.CityName
              FROM City AS c),
          null,
          Func<QueryContext, DbDataReader, ResultContext, SingleQueryResultCoordinator, IEnumerable<char>>,
          EFConcat.TestContext,
          False,
          False,
          True
      )'
dbug: 2/12/2024 10:32:10.415 RelationalEventId.CommandExecuting[20100] (Microsoft.EntityFrameworkCore.Database.Command)
      Executing DbCommand [Parameters=[], CommandType='Text', CommandTimeout='30']
      SELECT [c].[CityName]
      FROM [City] AS [c]

[roji]

This is likely our primitive collection support treating the string as a collection, and then not being able to deal with the literal string as such.

Even though this worked, this seems like quite a questionable way to express a string concatenation (at the very least, it's extremely inefficient in .NET itself). At least as a workaround, change this to simply .Select(itm => itm.CityName + "- A string") - the concatenation operator is a better way of doing this.

Beyond that, we can look at not treating strings as primitive collections in the query pipeline.