C# Generic Method Without Specifying Type

Ian Dallas picture Ian Dallas · Feb 12, 2011 · Viewed 13.3k times · Source

Ok so I'm a Java guy starting to use C# and I was coding and started making a generic method and what I wrote runs and compiles but it goes against everything I know about how generics should work so I'm hoping someone can explain this to me:

So I have a generic method defined as follows:

public static List<T> CopyAsList<T>(IEnumerable<T> list, Object lockObject)  
{  
    if (list != null)  
    {  
        lock (lockObject)  
        {  
            return new List<T>(list);  
        }  
    }  
    return null;  
}  

But the weird thing to me is that I can call this generic method without ever specifying T and it will work:

List<String> strings = new List<string>() { "a", "b", "c"};
List<int> ints = new List<int>() { 1,2,3};
object lockObject = new object();

foreach (string s in CopyAsList(strings, lockObject))
{
    Console.WriteLine(s);
}

foreach (int i in CopyAsList(ints, lockObject))
{
    Console.WriteLine(i);
}

How is it the code is able to compile without ever specifying the generic type? Does C# infer the type at runtime?

Answer

Femaref picture Femaref · Feb 12, 2011

No, it is inferred at compile time - the generic type parameter in the IEnumerable<T> you supply is used, which is known at compile time. Generally put, everything concerning generics and type parameters is specified at compile time. If there is mismatch of any kind, the compiler will complain and your code won't compile.

There are edge cases where you have to specify the types explicitly, these only occurs in rare circumstances with overloaded methods, sometimes with multiple combinations of type parameters.