Null propagation cannot be used for primitive type lists (string)

[frankiDotNet]

I want to parse a null propagation expression for a list with a string:

Simple test scenario:

public class TestClass{
    public List<string> MyList {get; set;}
}

var dataSource = new TestClass();
// Let MyList null...

var expressionText = "np(myList.FirstOrDefault())"
LambdaExpression e = DynamicExpressionParser.ParseLambda(ParsingConfig.Default, dataSource.GetType(),
               typeof(object), expressionText, null);
            var result = (e.Compile()).DynamicInvoke(dataSource);

Here I get the exception : The 'np' (null-propagation) function requires the first argument to be a MemberExpression

Null propagation for a property works:

var expressionText = "np(myList.Count)"
LambdaExpression e = DynamicExpressionParser.ParseLambda(ParsingConfig.Default, dataSource.GetType(),
               typeof(object), expressionText, null);
            var result = (e.Compile()).DynamicInvoke(dataSource);

[StefH]

Can you try MyGet version System.Linq.Dynamic.Core.1.0.23-ci-13135 ?

[frankiDotNet]

Looks good:

          var dataSource = new TestClass();
          
         var expressionText = "np(MyList.FirstOrDefault())";
         LambdaExpression e = DynamicExpressionParser.ParseLambda(ParsingConfig.Default, true, typeof(TestClass),
            typeof(object), expressionText, dataSource);
         var result = (e.Compile()).DynamicInvoke(dataSource);

{Param_0 => Convert(IIF(((Param_0 != null) AndAlso (Param_0.MyList != null)), Param_0.MyList .FirstOrDefault(), null))}

[StefH]

#368

[StefH]

@Franki1986 Can you do some more tests please? To make sure it works also on real database calls?

[frankiDotNet]

Sure, I will do it tomorow. But I am using Linqtodb not ef core. But if the expression is right I think it will also work with ef..

[frankiDotNet]

I am not sure what kind of test scenario I could do against a database? If I understand, I have to generate a methodExpression that could return null.

[StefH]

Instead of using DynamicExpressionParser manually, you can just use a Linq Select:

Example:

var resultDynamic = DBContext.MyClasses.AsQueryable().Select("np(MyList.FirstOrDefault())");

[frankiDotNet]

I hope this is getting in the right direction, because it is the closest I can create with the database and LinqToDb, because a string list cannot be mapped directly to a database table and the expression then coould only be applied to the resulting data. So I created a parent and child class for 1 to n.

public class Parent{
    public int Id {get; set;}
    public List<Child>ChildList {get; set;}
}

public class Child{
    public int Id {get; set;}
}

       using (var db = new DbContext())
       {
            var query = db.GetTable<Parent>().LoadWith(g => g.ChildList);
            var resultA = query .Select(c => c.ChildList.FirstOrDefault() != null).ToList();

            var query2 = query .Select("r => np(r.ChildList.FirstOrDefault()) != null", null);
            var result2 = query2 .ToDynamicList();
       }

Resulting expression debug string:

"{Table(PARENT).LoadWith(g => g.ChildList).Select(Param_0 => (IIF(((Param_0 != null) AndAlso (Param_0.ChildList!= null)), Param_0.ChildList.FirstOrDefault(), null) != null))}";

The result list is the same..

[StefH]

And if you get the Id and provide a default value, like:

var query2 = query .Select("np(r.ChildList.FirstOrDefault().Id, 42)");

[frankiDotNet]

Sorry for the late response, I wasn't at the pc over the easter days.
So this

var query2 = qu.Select("np(ChildList.FirstOrDefault().Id, 42)", null);

produces the expression:

{Table(PARENT).LoadWith(g => g.ChildList).Select(Param_0 => IIF((((Param_0 != null) AndAlso (Param_0.ChildList != null)) AndAlso (Param_0.ChildList.FirstOrDefault() != null)), Param_0.ChildList.FirstOrDefault().Id, 42))}

which returns 42 for Ids that would return null.

Works!
Will there be a direct nuget release or do you put this change in a future release?