C# – Casting between classes that share the same interface

ccastinginterface

I have two interfaces IHeaderRow, and IDetailRow

I then have an object that implements both RawRow:IHeaderRow, IDetailRow

I then need to cast it to HeaderRow which implements IHeaderRow.

But when I try, it ends up being null or giving an exception.

I can cast ObjectRawRow to either interface IHeaderRow, or IDetailRow

var ObjectIHeaderRow = ObjectRawRow as IHeaderRow;
var ObjectIDetailRow = ObjectRawRow as IDetailRow;

But I can not cast ObjectRawRow to HeaderRow , or ObjectIHeaderRow to HeaderRow.

It throws the error Cannot convert source type 'IA' to target type 'A'

I need to cast it into the actual class HeaderRow.

Thoughts?

EDIT:

Even though setting up an explicit cast took care of the issue I thought I'd provide an answer to the people wondering, WHY I was doing what I was.

In short, I'm sequentially processing a file. Line by line. I read the row into RawRow, and until I look at a few values, I don't actually know what type of row it is going to be. I then wanted to cast it to the proper type.

Best Answer

You can only implicitly cast objects to types they inherit from or implement - since RawRow doesn't derive from HeaderRow, it's not possible.

Depending on your requirements, you could overcome this by writing an explicit conversion operator, creating a HeaderRow constructor that accepts a RawRow as its prototype, or by modifying your code to operate on an IHeaderRow.

This is the style that Microsoft tends to use in their examples.

It appears that the guidance in this area may have changed, as StyleCop now enforces the use of the C# specific aliases.

C# – the difference between const and readonly in C#

Apart from the apparent difference of

having to declare the value at the time of a definition for a const VS readonly values can be computed dynamically but need to be assigned before the constructor exits.. after that it is frozen.
const's are implicitly static. You use a ClassName.ConstantName notation to access them.

There is a subtle difference. Consider a class defined in AssemblyA.

public class Const_V_Readonly
{
  public const int I_CONST_VALUE = 2;
  public readonly int I_RO_VALUE;
  public Const_V_Readonly()
  {
     I_RO_VALUE = 3;
  }
}

AssemblyB references AssemblyA and uses these values in code. When this is compiled:

in the case of the const value, it is like a find-replace. The value 2 is 'baked into' the AssemblyB's IL. This means that if tomorrow I update I_CONST_VALUE to 20, AssemblyB would still have 2 till I recompile it.
in the case of the readonly value, it is like a ref to a memory location. The value is not baked into AssemblyB's IL. This means that if the memory location is updated, AssemblyB gets the new value without recompilation. So if I_RO_VALUE is updated to 30, you only need to build AssemblyA and all clients do not need to be recompiled.

So if you are confident that the value of the constant won't change, use a const.

public const int CM_IN_A_METER = 100;

But if you have a constant that may change (e.g. w.r.t. precision).. or when in doubt, use a readonly.

public readonly float PI = 3.14;

Update: Aku needs to get a mention because he pointed this out first. Also I need to plug where I learned this: Effective C# - Bill Wagner

Best Answer

Related Solutions

C# – the difference between String and string in C#

This is the style that Microsoft tends to use in their examples.

C# – the difference between const and readonly in C#

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