Ternary expression and let with external boolean variable with false value produces invalid SQL.

[dmitryshunkov]

Let operator with expression that evaluates to false due to external argument in combination with conditional operator produces wrong SQL.

SELECT CASE
    WHEN 0
    THEN 1 ELSE 0
END
FROM [Posts] AS [post]

Exception message: An expression of non-boolean type specified in a context where a condition is expected, near 'THEN'.
Stack trace: at System.Data.SqlClient.SqlConnection.OnError(SqlException exception, Boolean breakConnection, Action`1 wrapCloseInAction)
   at System.Data.SqlClient.TdsParser.ThrowExceptionAndWarning(TdsParserStateObject stateObj, Boolean callerHasConnectionLock, Boolean asyncClose)
   at System.Data.SqlClient.TdsParser.TryRun(RunBehavior runBehavior, SqlCommand cmdHandler, SqlDataReader dataStream, BulkCopySimpleResultSet bulkCopyHandler, TdsParserStateObject stateObj, Boolean& dataReady)
   at System.Data.SqlClient.SqlDataReader.TryConsumeMetaData()
   at System.Data.SqlClient.SqlDataReader.get_MetaData()
   at System.Data.SqlClient.SqlCommand.FinishExecuteReader(SqlDataReader ds, RunBehavior runBehavior, String resetOptionsString)
   at System.Data.SqlClient.SqlCommand.RunExecuteReaderTds(CommandBehavior cmdBehavior, RunBehavior runBehavior, Boolean returnStream, Boolean async, Int32 timeout, Task& task, Boolean asyncWrite, SqlDataReader ds)
   at System.Data.SqlClient.SqlCommand.ExecuteReader(CommandBehavior behavior)
   at Microsoft.EntityFrameworkCore.Storage.Internal.RelationalCommand.Execute(IRelationalConnection connection, String executeMethod, IReadOnlyDictionary`2 parameterValues, Boolean closeConnection)
   at Microsoft.EntityFrameworkCore.Storage.Internal.RelationalCommand.ExecuteReader(IRelationalConnection connection, IReadOnlyDictionary`2 parameterValues)
   at Microsoft.EntityFrameworkCore.Query.Internal.QueryingEnumerable.Enumerator.BufferlessMoveNext(Boolean buffer)
   at Microsoft.EntityFrameworkCore.Storage.Internal.SqlServerExecutionStrategy.Execute[TState,TResult](Func`2 operation, Func`2 verifySucceeded, TState state)
   at Microsoft.EntityFrameworkCore.Query.QueryMethodProvider.<_ShapedQuery>d__3`1.MoveNext()
   at System.Linq.Enumerable.SelectEnumerableIterator`2.MoveNext()
   at Microsoft.EntityFrameworkCore.Query.Internal.LinqOperatorProvider.ExceptionInterceptor`1.EnumeratorExceptionInterceptor.MoveNext()
   at System.Collections.Generic.List`1..ctor(IEnumerable`1 collection)
   at System.Linq.Enumerable.ToList[TSource](IEnumerable`1 source)
   at TestEF.Program.Main() in C:\Git\Study\TestEF\TestEF\Program.cs:line 70

using System;
using System.Collections.Generic;
using System.Linq;
using Microsoft.EntityFrameworkCore;
using Microsoft.Extensions.Logging;

namespace TestEF
{
    public class BloggingContext : DbContext
    {
        public DbSet<Blog> Blogs { get; set; }
        public DbSet<Post> Posts { get; set; }

        protected override void OnConfiguring(DbContextOptionsBuilder optionsBuilder)
        {
            ILoggerFactory loggerFactory = new LoggerFactory()
                .AddDebug();

            //optionsBuilder.UseInMemoryDatabase();
            optionsBuilder.EnableSensitiveDataLogging();
            optionsBuilder.UseLoggerFactory(loggerFactory);
            optionsBuilder.UseSqlServer(@"Server=(localdb)\mssqllocaldb;Database=WonkyVB;Trusted_Connection=True;");
        }
    }

    public class Blog
    {
        public int Id { get; set; }
        public string Title { get; set; }

        public ICollection<Post> Posts { get; set; }
    }

    public class Post
    {
        public int Id { get; set; }
        public string Title { get; set; }
        public DateTime DateTime { get; set; }

        public int? BlogId { get; set; }
        public Blog Blog { get; set; }
    }

    public class PostDTO
    {
        public DateTime DateTime { get; set; }
        public string Title { get; set; }
    }

    public class Program
    {
        public static void Main()
        {
            using (var context = new BloggingContext())
            {
                context.Database.EnsureDeleted();
                context.Database.EnsureCreated();

                context.Add(new Blog
                {
                    Posts = new List<Post> { new Post { Title = "First" }, new Post { Title = "Second" }, new Post { Title = "Third" } }
                });
                context.SaveChanges();
            }

            using (var context = new BloggingContext())
            {
                bool externalCondition = false;

                var list = (from post in context.Posts
                           //let projectedExternalCondition = externalCondition                         // works
                           //let flag = post.Title == "some title" && projectedExternalCondition        // works
                           let flag = post.Title == "some title" && externalCondition                   // fails
                           select new
                           {
                               result = flag ? 1 : 0
                           }).ToList();

                foreach (var post in list)
                {
                    Console.WriteLine(post.result);
                }
            }
        }
    }
}

Further technical details

EF Core version: 1.1.2
Database Provider: Microsoft.EntityFrameworkCore.SqlServer
Operating system: Windows 10
IDE: Visual Studio 2017

[dmitryshunkov]

Seems to be fixed in 2.0.0-preview2-final.

[smitpatel]

Fixed via #7554