Cast generic type parameter to a specific type in C#

You can use Convert.ChangeType

SomeClass obj2 = (SomeClass)Convert.ChangeType(t, typeof(SomeClass));

Although, keep in mind that this will throw an exception if a cast is invalid.

Using as:

SomeClass obj2 = t as SomeClass;

This would not throw an exception and t would be null if the cast fails.

I don't really know what you're trying to do, but I hope that you're not missing the point of Generics here.

If your intention is to restrict the method to type SomeClass and descendants:

void SomeMethod(T t)  where T : SomeClass

A better design is to put a constraint on it that is common between type T and the class you want to expect in your method, in this case SomeClass.

class SomeConsumer<T> where T : ISomeClass
{
    void SomeMethod(T t)
    {
        ISomeClass obj2 = (ISomeClass) t;
    }
}

interface ISomeClass{}

class SomeClass : ISomeClass {}

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Edit based on edit of Question

That is bad design. Try to move that "operation" into the class itself so the caller does not have to know the type. If that is not possible share more of what is being done, what you want to accomplish though is that you do not have a stack of if/else statements where execution depends on the type of object being passed in to the method.

class SomeConsumer<T> where T : ISomeClass
{
    void SomeMethod(T t)
    {
        ISomeClass obj2 = (ISomeClass) t;
        // execute
        t.Operation();
    }
}

interface ISomeClass{
    void Operation();
}

class SomeClass : ISomeClass {
    public void Operation(){/*execute operation*/}
}