Question

Please explain the following error on struct constructor. If i change struct to class
the erros are gone.

public struct DealImportRequest
{
    public DealRequestBase DealReq { get; set; }
    public int ImportRetryCounter { get; set; }

    public DealImportRequest(DealRequestBase drb)
    {
        DealReq = drb;
        ImportRetryCounter = 0;
    }
}

• error CS0188: The ‘this’ object cannot be used before all of its fields are assigned to
• error CS0843: Backing field for automatically implemented property
  ‘DealImportRequest.DealReq’ must be fully assigned before control is returned to the caller. Consider calling the default constructor from a constructor initializer.

Answer 1

As the error message recommends, you can resolve this by calling the default constructor from a constructor initializer.

public DealImportRequest(DealRequestBase drb) : this()
{
   DealReq = drb;
   ImportRetryCounter = 0;
}

From the language specification:

10.7.3 Automatically implemented properties

When a property is
specified as an automatically
implemented property, a hidden backing
field is automatically available for
the property, and the accessors are
implemented to read from and write to
that backing field. […] Because the
backing field is inaccessible, it can
be read and written only through the
property accessors, even within the
containing type. […] This
restriction also means that definite
assignment of struct types with
auto-implemented properties can only
be achieved using the standard
constructor of the struct, since
assigning to the property itself
requires the struct to be definitely
assigned. This means that user-defined
constructors must call the default
constructor.

The other (more verbose) alternative, of course, is to manually implement the properties and set the backing fields yourself in the constructor.

Do note that the struct you have there is mutable. This is not recommended. I suggest you either make the type a class (your compilation problems should go away immediately) or make the type immutable. The easiest way to accomplish this, assuming the code you have presented is the entire struct, would be to make the setters private (get; private set;). Of course, you should also make sure that you don’t add any mutating methods to the struct afterwards that rely on private access to modify the fields. Alternatively, you could back the properties with readonly backing fields and get rid of the setters altogether.