nullchecks.cs

// good old if-then-throw
if (foo == null)
  throw new ArgumentNullException("foo");

// nameof-flavored if-then-throw
if (foo == null)
  throw new ArgumentNullException(nameof(foo));

// reference equality to avoid operator== overload
if (ReferenceEquals(foo, null))
  throw new ArgumentNullException(nameof(foo));

// one more way to avoid operator== overload
if ((object) foo == null)
  throw new ArgumentNullException(nameof(foo));

// and one more way
if (!(foo is object))
  throw new ArgumentNullException(nameof(foo));

// code contracts
Contract.Requires(foo != null);

// debug assert
Debug.Assert(foo != null);
Trace.Assert(foo != null)

// user-defined util
Require.NotNull(foo);
Check.NotNull(foo, nameof(foo));

// C# 7.0 throw-expression powered way
foo = foo ?? throw new ArgumentNullException(nameof(foo));