EF Core using Let in query throw "Argument types do not match" exception

Question

EF Core using Let in query throw "Argument types do not match" exception

c# entity-framework entity-framework-core linq

Question

I'm using EF Core 2.0.1. I'm trying to get the last order data along with a user info to populate UserDto as follows,

(from u in DbContext.User.Where(u => u.UserId == userId)
let last = u.Orders.LastOrDefault()
select new UserDto
{
    Id = u.Id,
    Name = u.Name,
    LastOrderId = (last == null ? null : last.Id),
    LastOrderDate = (last == null ? null : last.Date)
}

This throws following exception. How can I fix this?

at System.Linq.Expressions.Expression.Condition(Expression test, Expression ifTrue, Expression ifFalse, Type type)
at System.Linq.Expressions.ConditionalExpression.Update(Expression test, Expression ifTrue, Expression ifFalse)
at System.Linq.Expressions.ExpressionVisitor.VisitConditional(ConditionalExpression node)
at System.Linq.Expressions.ConditionalExpression.Accept(ExpressionVisitor visitor)
at System.Linq.Expressions.ExpressionVisitor.Visit(Expression node)
at Microsoft.EntityFrameworkCore.Query.ExpressionVisitors.RelationalProjectionExpressionVisitor.Visit(Expression expression)
at System.Linq.Expressions.ExpressionVisitor.VisitMemberAssignment(MemberAssignment node)
at System.Linq.Expressions.ExpressionVisitor.VisitMemberBinding(MemberBinding node)
at System.Linq.Expressions.ExpressionVisitor.Visit[T](ReadOnlyCollection`1 nodes, Func`2 elementVisitor)
at System.Linq.Expressions.ExpressionVisitor.VisitMemberInit(MemberInitExpression node)
at Microsoft.EntityFrameworkCore.Query.ExpressionVisitors.RelationalProjectionExpressionVisitor.VisitMemberInit(MemberInitExpression memberInitExpression)
at System.Linq.Expressions.MemberInitExpression.Accept(ExpressionVisitor visitor)
at System.Linq.Expressions.ExpressionVisitor.Visit(Expression node)
at Microsoft.EntityFrameworkCore.Query.ExpressionVisitors.RelationalProjectionExpressionVisitor.Visit(Expression expression)
at Microsoft.EntityFrameworkCore.Query.EntityQueryModelVisitor.VisitSelectClause(SelectClause selectClause, QueryModel queryModel)
at Microsoft.EntityFrameworkCore.Query.RelationalQueryModelVisitor.VisitSelectClause(SelectClause selectClause, QueryModel queryModel)
at Remotion.Linq.Clauses.SelectClause.Accept(IQueryModelVisitor visitor, QueryModel queryModel)
at Remotion.Linq.QueryModelVisitorBase.VisitQueryModel(QueryModel queryModel)
at Microsoft.EntityFrameworkCore.Query.EntityQueryModelVisitor.VisitQueryModel(QueryModel queryModel)
at Microsoft.EntityFrameworkCore.Query.RelationalQueryModelVisitor.VisitQueryModel(QueryModel queryModel)
at Microsoft.EntityFrameworkCore.Query.EntityQueryModelVisitor.CreateAsyncQueryExecutor[TResult](QueryModel queryModel)
at Microsoft.EntityFrameworkCore.Storage.Database.CompileAsyncQuery[TResult](QueryModel queryModel)
at Microsoft.EntityFrameworkCore.Query.Internal.QueryCompiler.CompileAsyncQueryCore[TResult](Expression query, INodeTypeProvider nodeTypeProvider, IDatabase database)
at Microsoft.EntityFrameworkCore.Query.Internal.QueryCompiler.<>c__DisplayClass24_0`1.<CompileAsyncQuery>b__0()
at Microsoft.EntityFrameworkCore.Query.Internal.CompiledQueryCache.GetOrAddQueryCore[TFunc](Object cacheKey, Func`1 compiler)
at Microsoft.EntityFrameworkCore.Query.Internal.CompiledQueryCache.GetOrAddAsyncQuery[TResult](Object cacheKey, Func`1 compiler)
at Microsoft.EntityFrameworkCore.Query.Internal.QueryCompiler.CompileAsyncQuery[TResult](Expression query)
at Microsoft.EntityFrameworkCore.Query.Internal.QueryCompiler.ExecuteAsync[TResult](Expression query)
at Microsoft.EntityFrameworkCore.Query.Internal.EntityQueryProvider.ExecuteAsync[TResult](Expression expression)
at Microsoft.EntityFrameworkCore.Query.Internal.EntityQueryable`1.System.Collections.Generic.IAsyncEnumerable<TResult>.GetEnumerator()
at System.Linq.AsyncEnumerable.<Aggregate_>d__6`3.MoveNext()

sajith

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Ivan Stoev · Accepted Answer · 3/31/2018 2:07:53 PM

Formally there is nothing wrong with your query. But although EF Core tries to translate Last / LastOrDefault methods (EF6 simply throws NotSupportedException), apparently it still has issues doing so.

The problem is that Last / LastOrDefault is not quite defined for database originating sequences which are unordered by nature.

Shortly, avoid Last / LastOrDefault. Use OrderByDescending + First / FirstOrDefault instead.

The following works:

let last = u.Orders.OrderByDescending(o => o.Date).FirstOrDefault()

Just use the ordering column(s) which is appropriate for your data logic for determining what is supposed to be "first", "last" etc.

Actually this also works:

let last = u.Orders.OrderBy(o => o.Date).LastOrDefault()

but EF Core will try to convert it to the aforementioned construct, thus is more prone to EF Core implementation bugs than the explicit way.

EF Core using Let in query throw "Argument types do not match" exception

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