使用反射传递参数时，Visual Studio强制转换是多余的

Question

With this line of code, VS2015 shows "Cast is redundant" for both arguments (but works!!).

Activator.CreateInstance(type, (IOperationContext)null, (Dictionary<string, object>)null);

Whereas if I remove them I get:

Activator.CreateInstance(type, null, null);

It throws " System.MissingMethodException " exception:

Constructor on type 'iBI.Services.Hsd.Operations.Schedule.RefreshUsedOperation' not found.

Is there something wrong with the usage or this is just false alram by visual studio?

Answer 1

When you call

Activator.CreateInstance(type, null, null)

it actually calls

CreateInstance(Type type, object[] args, object[] activationAttributes)

method overload and this CreateInstance overload looks for parameterless ctor which cannot be found.

When you call

Activator.CreateInstance(type, (IOperationContext)null, (Dictionary<string, object>)null);

then it calls

CreateInstance(Type type, params object[] args)

overload. And then ctor with two params found and called. Both params passed as null.

Answer 2

I would say it's a false alarm by VS, since reflection needs those casts to know which construction to call. Without those casts, the parameters you give have no types, which makes it impossible for the Activator to know which signature to use, especially if you have multiple constructors which take 2 parameters of different types. The warning probably comes from the fact that this would indeed be redundant in almost all other situation, since a null can fit in all reference types (and the compiler will throw you an error if you try to put a null in a value type) and will (kinda) take the type of its container. At this point, if you wish you could open a bug report with Visual Studio as this is indeed something that could (should?) be fixed.

Answer 3

Actually, Visual Studio is kind of correct in that having both casts is redundant. If you remove either one of the casts but leave the other one there, VS will stop complaining because you don't have any redundant casts, but will still have enough information in place to invoke the correct method overload:

Both of these options are fine:

Activator.CreateInstance(type, null, (Dictionary<string, object>)null);
Activator.CreateInstance(type, (OperationContext)null, null);

Answer 4

It's not problem with your IDE. Just open MSDN and look to overrides: this and this . When you wrote:

Activator.CreateInstance(type, (IOperationContext)null, (Dictionary<string, object>)null);

.NET know, that it should use the second method . When your wrote:

Activator.CreateInstance(type, null, null);

I think, .NET interprete it as first method , that's why you have problem. You can try to write next code:

Activator.CreateInstance(youType, args:null, activationAttributes:null);

And it should work.