You can restrict what types a generic can be by using constraints. Common constraints include:

• where T : class – T must be a reference type
• where T : struct – T must be a value type
• where T : new() – T must have a public parameterless constructor
• where T : BaseClass – T must inherit from a specific base class
• where T : Interface – T must implement a specific interface
• where T : unmanaged – T must be an unmanaged type (like int, float, etc.)

Example

public class Printer<T> where T : class
{
	public void Print(T item)
	{
		if (item == null)
		{
			Console.WriteLine("Nothing to print.");
			return;
		}
		Console.WriteLine(item.ToString());
	}
}

public void Main()
{
	Printer<string> p1 = new Printer<string>();
	p1.Print("Hello");      // prints "Hello"
	p1.Print(null);         // prints "Nothing to print"
	
	Printer<int> p2 = new Printer<int>(); // compile-time error: int is not a class
}