What is the difference between a generic type and a generic type definition?
I'm Studying up on .net reflection and am having a hard time figuring out the difference.
Fr开发者_运维知识库om what I understand, List<T>
is a generic type definition. Does that mean that to .net reflection T is the generic type?
Specifically, I guess I'm looking for more background on the Type.IsGenericType and Type.IsGenericTypeDefinition functions.
Thanks!
In your example List<T>
is a generic type definition. T
is called a generic type parameter. When the type parameter is specified like in List<string>
or List<int>
or List<double>
then you have a generic type. You can see that by running some code like this...
public static void Main()
{
var l = new List<string>();
PrintTypeInformation(l.GetType());
PrintTypeInformation(l.GetType().GetGenericTypeDefinition());
}
public static void PrintTypeInformation(Type t)
{
Console.WriteLine(t);
Console.WriteLine(t.IsGenericType);
Console.WriteLine(t.IsGenericTypeDefinition);
}
Which will print
System.Collections.Generic.List`1[System.String] //The Generic Type.
True //This is a generic type.
False //But it isn't a generic type definition because the type parameter is specified
System.Collections.Generic.List`1[T] //The Generic Type definition.
True //This is a generic type too.
True //And it's also a generic type definition.
Another way to get the generic type definition directly is typeof(List<>)
or typeof(Dictionary<,>)
.
This will help explain it maybe:
List<string> lstString = new List<string>();
List<int> lstInt = new List<int>();
if (lstString.GetType().GetGenericTypeDefinition() ==
lstInt.GetType().GetGenericTypeDefinition())
{
Console.WriteLine("Same type definition.");
}
if (lstString.GetType() == lstInt.GetType())
{
Console.WriteLine("Same type.");
}
If you run it you will get the first test to pass because both items are implementing the type List<T>
. The second test fails because List<string>
is not the same as List<int>
. The generic type definition is comparing the original generic before T
is defined.
The IsGenericType
type is just checking if the generic T
has been defined. IsGenericTypeDefinition
checks to see that the generic T
has NOT been defined. This is useful if you want to know if two objects have been defined from the same base generic type such as the first List<T>
example.
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