What does [param: NotNull] mean in C#?

Question

In Entity Framework's source code (link) I found this line:

public virtual IRelationalTransaction Transaction  { get; [param: NotNull] protected set; } 

The [param: NotNull] part looks very strange to me. Any idea what kind of a C# syntax is this? I'm familiar with attributes and param but not this combination.

The definition of NotNull is this:

[AttributeUsage(     AttributeTargets.Method | AttributeTargets.Parameter |     AttributeTargets.Property | AttributeTargets.Delegate |     AttributeTargets.Field)] internal sealed class NotNullAttribute : Attribute { } 

Which I expected to be used simply as [NotNull] but what is param doing here?

Answer 1

When you mark method with NotNull it means, that method returns not null object:

[NotNull] public object Get() {     return null; //error } 

When you mark setter it does the same - setter returns not null (because .net converts properties to get and set methods).

public virtual IRelationalTransaction Transaction { get; [NotNull] protected set; } 

Equals to:

[NotNull]  public virtual void set_Transaction(IRelationalTransaction value) { ... } 

So, you need to add param: to point, that "i mean - parameter of setter is not null, not a result of set-method":

public virtual IRelationalTransaction Transaction { get; [param: NotNull] protected set; } 

Equals to:

public virtual void set_Transaction([NotNull] IRelationalTransaction value) { ... }