C# constructing generic method

Question

var leftCurrent = leftArray.GetValue(i);
var rightCurrent = rightArray.GetValue(i);

var mi = typeof (PropertyCompare).GetMethod("NotEqualProperties");
mi.MakeGenericMethod(leftCurrent.GetType());

var notEqualProps = mi.Invoke(null,new []{leftCurrent, rightCurrent});

if(notEqualProps != null) 
    result.Add(new ArraysDiffResult(i, notEqualProps as List<string>));

Why does this code throws InvalidOperationException ( Late bound operations cannot be performed on types or methods for which ContainsGenericParameters is true.) ?

NotEqualProperties is static generic method..

UPD : I've already found solution. Just forgot to assign new MethodInfo...(Epic Fail..)

But how about performance?

Answer 1

MakeGenericMethod returns a new MethodInfo instance. (MethodInfo is immutable)

Your code creates this new instance, throws it away, then continues using the open (non-parameterized) MethodInfo.

You need to use the new instance, like this:

mi = mi.MakeGenericMethod(leftCurrent.GetType());

---

Yes; reflection is much slower than normal method calls.
However, unless you're calling it in a tight loop, it's not necessarily an issue.

Answer 2

You didn't assign the result of

mi.MakeGenericMethod(leftCurrent.GetType());

to anything. Note that MakeGenericMethod does not mutate the invoking instance.

P.S Is this code much slower than calling method directly (without mi.Invoke) ?

Much? I don't know. The only way to know is to set performance benchmarks and to profile.

Answer 3

Oh, I'm stupid...It should be :

mi = mi.MakeGenericMethod(leftCurrent.GetType());

(Facepalm...). But how about performance?