Why do I need “field:” in my attribute declaration “[field:NonSerialized]”?

Question

I can't find "field" listed as a C# keyword anywhere. Does anyone know the background on this?

Answer 1

This is necessary, for example, if you are marking an event as non-serializable. It specifies the target that the attribute applies to.

It is part of the attribute target syntax. From the specification :

attribute-target:
    field
    event
    method
    param
    property
    return
    type

See also the documentation for NonSerializedAttribute :

To apply the NonSerializedAttribute class to an event, set the attribute location to field, as shown in the following C# code.

[field:NonSerializedAttribute()]

public event ChangedEventHandler Changed;

Answer 2

The C# compiler usually has no trouble figuring out what part of a declaration the attribute applies to. I can think of three cases where you might use it:

1. Attributes that apply to the assembly. Very visible in AssemblyInfo.cs
2. An attribute applied to the return value of a P/Invoke declaration, [return:MarshalAs]
3. Having the attribute apply to the backing variable of a property or event without accessors. Your case.

Answer 3

This is meant to allow you to set NonSerialized attribute on fields, this is useful in serializing events.

For instance this would give you a compilation error

[NonSerialized]
public event SomeEventHandler SomeEvent;

To fix this you have to use field:

[field:NonSerialized]
public event SomeEventHandler SomeEvent;

More on this here -- Delegates and Serialization

Answer 4

The NonSerializedAttribute is only applicable to fields, you can use it as follows:

[NonSerialized]
public string myString;