Expression tree yields Argument exception

Question

I have the following piece of code, all works well until I get to the very last line where it fails with the following exception:

Method 'Boolean Contains(System.Linq.Expressions.ConstantExpression)' declared on type 'System.Collections.Generic.List`1[System.Linq.Expressions.ConstantExpression]' cannot be called with instance of type 'System.Guid'

var filter = new List<SomeObj> { new SomeObj { Id = "<guid-string>" }};

var lookupExpression = filter.SetOperand.Select(x => Expression.Constant(Guid.Parse(x.Id))).ToList();
var arrayOfValues = Expression.NewArrayInit(typeof(Guid), lookupExpression);
var arrayType = lookupExpression.GetType();
var containsMethod = arrayType.GetMethod("Contains");

var right = Expression.Call(dataProperty, containsMethod, arrayOfValues);

I think the problem is that dataProperty is read from a dynamically constructed expression which would always be a Guid so when the method executes, it sees this object as a Guid while the method and list are both List. Is there some other way around this?

The `Guid` data type cannot be `const`, see: http://stackoverflow.com/questions/4926573/how-to-declare-a-constant-guid-in-c — Matthew, Apr 28 '15 at 15:56
You can't call `filter.Contains(guid)` either way. But `filter.Any(f => f.Id == guid)`. — abatishchev, Apr 28 '15 at 15:57
@abatishchev how do you recommend I convert this to a List filter.SetOperand.Select(x => Expression.Constant(Guid.Parse(x.Id))); Which is required for Epression.NewArrayInit. Or are you recommending a totally different approach? — Obi, Apr 28 '15 at 16:15
What is the type of `SomeObj.id`? Is it string containing guid or System.Guid? — abatishchev, Apr 28 '15 at 16:18
You can also use `filter.Select(f => f.id).Contains(guid)` but it would be less efficient I think. — abatishchev, Apr 28 '15 at 16:19

score 2 · Accepted Answer · answered Apr 28 '15 at 18:11

I don't quite understand what are you trying to do or why, but here is my guess how to fix your code:

You don't want to check that your collection of expressions of GUIDs (lookupExpression) contains a given GUID, you want to check that your expression of collection of GUIDs (arrayOfValues) does. This means that arrayType is wrong, it should be: var arrayType = arrayOfValues.Type;.
If arrayOfValues is actually meant to be array, you can't use a Contains instance method, because array doesn't have one.

You can either use LINQ Contains, or change arrayOfValues to be an expression that represents List<Guid> instead of Guid[]. I have chosen LINQ.

To get the LINQ Contains method, you can use LINQ:
```
var containsMethod = typeof(Enumerable).GetMethods()
    .Single(m => m.Name == "Contains" && m.GetParameters().Length == 2)
    .MakeGenericMethod(typeof(Guid));
```
Your Call() is in the wrong order even for List.Contains(). For LINQ Contains, the correct order is:
```
Expression.Call(containsMethod, arrayOfValues, dataProperty)
```

I strongly suspected this would be the case as I have since juggled my code around to almost look like what you have here but this has answered all the nagging questions I had. I will try it when I get in. Thanks — Obi, Apr 29 '15 at 04:41

Expression tree yields Argument exception

1 Answers1