NotNullAttribute
Specifies that an output will not be null even if the corresponding type allows it. Specifies that an input argument was not null when the call returns.
namespace System.Diagnostics.CodeAnalysis
{
[AttributeUsage(AttributeTargets.Property | AttributeTargets.Field | AttributeTargets.Parameter | AttributeTargets.ReturnValue, Inherited = false)]
internal sealed class NotNullAttribute : Attribute
{
}
}