检查值和引用类型是否为“ null”

Question

I'm currently a little confused about checking for 'null' (empty, not available, without value) on value and or reference types. There seem to be a lot of possible ways to do so and all of them are frequently used.

Let's say I've got the following extension method which can be used somehow similar to a maybe monad:

public static TResult UseWith<T, TResult>(this T value, Func<T, TResult> action)
{
    // if value not null
    //     execute action
    // else return the default value of 'TResult'
}

So? How do I check for null?

if(value != null)

or (which checks for null on reference types, but for default on value types)

if(!Equals(value, default(T))

or

if(!EqualityComparer<T>.Default.Equals(value, default(T))

or (with precheck)

 if (!typeof(T).IsValueType)
 {
     if (Equals(source, default(T))) 
          return TResult; 
 }

Or is it better to create 2 methods with restrictions?

 public static TResult UseWith<T, TResult>(this T value, Func<T, TResult> action)
    where T : class
 {
    if(value == null)

  public static TResult UseWith<T, TResult>(this T value, Func<T, TResult> action)
    where T : struct
 {
    if(value.HasValue)

Answer 1

The pseudocode in your first snippet will simply work exactly as you want it to. A variable that is typed as an unbounded generic argument can be compared to null using the == operator. The code will compile, and it will simply evaluate to false whenever T is a non-nullable value type.

You don't want to compare T to the default value, because the default value may not be null .

There is no need to explicitly check if T is a value type, or have separate overloads for reference/value types, as the == operator handles this when it's JITted.