Error creating Null string

[rockResolve]

When using "select string(null)" I get the "Ambiguous invocation of 'String' constructor" error.
I was able to use this select in the old kahanu/System.Linq.Dynamic.

I need the String cast because I Concat this with a query with another and the Concat insists on matching types.

How can I get a Select to return a null string from a literal null?

[StefH]

See also this one:
#284
Doe this solve your issue?

[rockResolve]

No. Constants.IsNull sees the value as "null" and returns false

[rockResolve]

When I use select string(null) in the old kahanu/System.Linq.Dynamic it works.

Diagnosis: The difference is in MethodFinder.cs IsBetterThan.

The string constructor has 8 methods, FindBestMethod finds 3 methods (Char*, SByte* & Char[]) initially applicable.
kahanu IsBetterThan returns false when CompareConversion finds the parms are equal
So FindBestMethod returns final applicable.Length = 0 and the calling ParseTypeAccess falls back to GenerateConversion which succeeds.

However System.Linq.Dynamic.Core IsBetterThan returns true when the parms are equal.
So FindBestMethod returns final applicable.Length = 3 and the calling ParseTypeAccess throws the exception.

Not sure how you would want to fix this.

[StefH]

@rockResolve
Can you try MyGet version System.Linq.Dynamic.Core.1.0.20-ci-12452.nupkg ?

[StefH]

Actually using code like string(null) should not call the new operator, it's just a cast.
Also when casting to nullable int, you would use : int?(5).

[rockResolve]

Thanks for the quick reply. The package didn't quite work for me.
I am using the results with a concat, so need string(null), not just null. 

So I downloaded the branch to look closer. After I removed the "If only 1 argument, and the arg is ConstantExpression, just return the ConstantExpression" code, your pointer constructor removal worked well and built my string(null).
Now all my tests pass, which include testing nullable bools, ints, dates.

Are you able to remove the "If only 1 argument, and the arg is ConstantExpression, just return the ConstantExpression" code or is it there for another reason?

[StefH]

When you remove that line, the dynamic code behaves like this normal c# code:
new string((char[]) null); and will create an empty string, which differs from a null string!

If you need concat, can't you just use string(\"\") ?

[rockResolve]

Oops your right my test was falsely passing with an empty string.

However I still do really need a string(null).
I am using linq.concat (like tsql union) to join several sets, but concat insists on matching types.
I am using string(null) to ensure this set is sorted first/last

[StefH]

Can you provide an simple console-app?

[rockResolve]

Finally got a simple console-app made.
I had to use z.entityframework to get my group by working correctly in core.
What is the best way to share - a PR in this repo?

[StefH]

Just create a new project on github.

If needed I can copy it and debug in my solution.

[rockResolve]

Added console-app at https://github.com/rockResolve/dlinq-demo-string-null.
Currently using System.Linq.Dynamic.Core 1.0.20-ci-12519, so string(null) raises pointer constructor error.

[rockResolve]

Also added possible System.Linq.Dynamic.Core fix with added/updated tests at:
https://github.com/rockResolve/System.Linq.Dynamic.Core/tree/Better_Error-null-string

[StefH]

Can you create a PR, I'll have to check your changes.

[rockResolve]

PR #354
I based it on the latest master, to pick up your latest changes

[StefH]

PR is merged ; closing this issue.