System.Diagnostics.CodeAnalysis.NotNullIfNotNullAttribute

sealed class NotNullIfNotNullAttribute : Attribute

Specifies that the output will be non-null if the named parameter is non-null.

public string ParameterName { get; }

Gets the associated parameter name.

public NotNullIfNotNullAttribute(string parameterName)

Initializes the attribute with the associated parameter name.