Call C# Method with Expression<Func> Parameter From Powershell

Question

I have the following sample C# class:

public static class Class1
{
    public static string Method1(Expression<Func<string>> efs)
    {
        return efs.Compile().Invoke();
    }
}

To call this from C# it's as easy as:

Class1.Method1(() => "Hello World");

I can not for the life of me figure out how to call this from Powershell. My last attempt is:

Add-Type -Path "ClassLibrary1.dll"
$func = [Func[string]] { return "Hello World" }
$exp = [System.Linq.Expressions.Expression]::Call($func.Method);
[ClassLibrary1.Class1]::Method1($exp)

But this results in an error:

Exception calling "Call" with "1" argument(s): "Incorrect number of arguments supplied for call to method 'System.String lambda_method(System.Runtime.CompilerServices.Closure)'"
At C:\Users\Mark\Documents\Visual Studio 2015\Projects\ClassLibrary1\ClassLibrary1\test.ps1:4 char:1
+ $exp = [System.Linq.Expressions.Expression]::Call($func.Method);
+ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    + CategoryInfo          : NotSpecified: (:) [], MethodInvocationException
    + FullyQualifiedErrorId : ArgumentException
+ $exp = [System.Linq.Expressions.Expression]::Call($func.Method);
+ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    + CategoryInfo          : NotSpecified: (:) [], MethodInvocationException
    + FullyQualifiedErrorId : ArgumentException

I believe my $func is incorrect; any ideas?

I don't see an overload of Expression.Call that takes a single argument. That is what the error is telling you as well. — Jason Boyd
– Jason Boyd, Commented Aug 3, 2017 at 15:47
@DavidG, no, that results in Cannot convert argument "efs", with value: "System.Func1[System.String]", for "Method1" to type "System.Linq.Expressions.Expression1[System.Func1[System.String]]", which is why I was trying to create an Expression` from the Func. — Mark
– Mark, Commented Aug 3, 2017 at 15:55
@JasonBoyd, good point, when I type the analogous in C# Func<string> func = () => "Hello World"; Expression.Call(func.Method);, it's using the signature: public static MethodCallExpression Call(MethodInfo method, params Expression[] arguments), now I'm stuck trying to figure out how to call that... — Mark
– Mark, Commented Aug 3, 2017 at 15:58
That's interesting. When I try that in LinqPad I get Static method requires null instance, non-static method requires non-null instance.Parameter name: method — Jason Boyd
– Jason Boyd, Commented Aug 3, 2017 at 16:15
You're also trying to convert a Func to an Expression which isn't really possible. Does it need to be an Expression? Because that would work very simply. — DavidG
– DavidG, Commented Aug 3, 2017 at 16:17

Mark · Accepted Answer · 2017-08-03 17:27:12Z

4

Ok, figured it out (thanks to @DavidG for the link). The key was to write out the System.Linq.Expressions.Expression tree in C# first. Then, converting to Powershell was easy after that:

So, in C# this:

Class1.Method1(() => "Hello World");

is analogous to:

var exp = Expression.Constant("Hello World", typeof(string));
var lamb = Expression.Lambda<Func<string>>(exp);
Class1.Method1(lamb);

In Powershell:

$exp = [System.Linq.Expressions.Expression]::Constant("Hello World", [string]);
$lamb = [System.Linq.Expressions.Expression]::Lambda([Func[string]], $exp);
[ClassLibrary1.Class1]::Method1($lamb);

edited Aug 3, 2017 at 17:27

answered Aug 3, 2017 at 17:19

Mark

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