How to use local variable as a type? Compiler says "it is a variable but is used like a type"

Question

At run-time, I don't know what type of variable v1 is. For this reason, I wrote many if else statements:

if (v1 is ShellProperty<int?>)
{
    v2 = (v1 as ShellProperty<int?>).Value;
}
else if (v1 is ShellProperty<uint?>)
{
    v2 = (v1 as ShellProperty<uint?>).Value;
}
else if (v1 is ShellProperty<string>)
{
    v2 = (v1 as ShellProperty<string>).Value;
}
else if (v1 is ShellProperty<object>)
{
    v2 = (v1 as ShellProperty<object>).Value;
}

The only difference is in ShellProperty<AnyType>.

So instead of writing this with a lot of if else statements, I decided to use reflection to get the property type at run-time:

 Type t1 = v1.GetType().GetProperty("Value").PropertyType;
 dynamic v2 = (v1 as ShellProperty<t1>).Value;

This code gets the PropertyType of v1 and assigns it to the local variable t1, but after that, my compiler says that:

t1 is a variable but is used like a type

So it does not allow me to write t1 inside ShellProperty<>.

Please tell me how to solve this problem and how to get more compact code than what I have. Do I need to create a new class?

You can't use generics in that way. Generics are strongly typed at compile time. — David L, Oct 04 '15 at 15:48
not statically like this. but you are already doing dynamic. — Daniel A. White, Oct 04 '15 at 15:51
Not using dyanmic objects. If your value is an object (i.e. inherits from System.Object or better yet some base class that you own), you could do `object v2 = v1 as ShellProperty`. (Again, using your own base class--or better yet an interface--would make this more useful.) — devstruck, Oct 04 '15 at 15:53

OxCantEven · Accepted Answer · 2015-10-04T16:16:23.517

29

You were very close, you were just missing a call to MakeGenericType.

I believe your code would look like the following:

Type t1 = v1.GetType().GetProperty("Value").PropertyType;
var shellPropertyType = typeof(ShellProperty<>);
var specificShellPropertyType = shellPropertyType.MakeGenericType(t1);
dynamic v2 = specificShellPropertyType.GetProperty("Value").GetValue(v1, null);

Edit: As @PetSerAl pointed out I added some layers of indirection that were unnecessary. Sorry OP, you probably want a one liner like:

dynamic v2 = v1.GetType().GetProperty("Value").GetValue(v1, null);

edited Oct 04 '15 at 16:16

answered Oct 04 '15 at 15:55

OxCantEven

629
4
9

2

Is that: `v1.GetType().GetProperty("Value").GetValue(v1, null)` not enough? – user4003407 Oct 04 '15 at 16:07
1

@PetSerAl You're absolutely right, I feel very silly. I spent too much time looking at the original code and not enough time thinking about my answer. I will update now. – OxCantEven Oct 04 '15 at 16:14
2

very funny! maybe @PetSerAl is right! But anyway this was for "How to use Local variable as a type?" – IremadzeArchil19910311 Oct 05 '15 at 07:00
1

I believe this was not for the question "How to use Local variable as a type?" as i get the same error... – Ricardo Araújo Jan 04 '22 at 19:36

score 16 · Answer 2 · edited May 23 '17 at 12:08

For generics, you have to create them dynamically.

MethodInfo method = typeof(Sample).GetMethod("GenericMethod");
MethodInfo generic = method.MakeGenericMethod(myType);
generic.Invoke(this, null);

To create a generic object, you can

var type = typeof(ShellProperty<>).MakeGenericType(typeof(SomeObject));
var v2 = Activator.CreateInstance(type);

Please refer to Initializing a Generic variable from a C# Type Variable

How to use local variable as a type? Compiler says "it is a variable but is used like a type"

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