C# Tips: Keep it Short, Get it Done - Using “as” and “is"
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The “as” operator provides a semantically logical and programmatically safe type cast. One of the more common situations requiring explicit casts is when working with ASP.NET web controls and navigating the control hierarchy. In the following example, we need to convert a web control’s “Parent” reference to its proper type, in this case a “DataGridItem.”
Using C-style casts:
try
{
DataGridItem d = (DataGridItem)obj.Parent;
if (d.GetType() == “DataGridItem”)
{
// Do something with d
}
}
catch (InvalidCastException e)
{
…
}
Using “as”
DataGridItem d = obj.Parent as DataGridItem;
if (null != d)
{
// Do something with d
}
Always use “as” in place of the traditional C-style cast. Do not use “as” in place of casting methods exposed by the object itself unless you are certain of the result. For example, you should not use “object as string” when you could use “object.ToString()” because it is assumed that the object knows better how to cast itself than the runtime.
The “is” operator is a native language construct that exposes an object’s type. “is” spares one method call in IL by avoiding the call to “Object.GetType()”. If you are familiar with Visual Basic, you will recognize this operator.
Using the example for the “as” operator, consider this statement:
DataGridItem d = obj.Parent as DataGridItem;
if (d is DataGridItem)
{
// Do something with d
}
The difference, as you probably noticed, is that in the “as” example d is compared for inequality against null, whereas in the “is” example it was compared against the type in question. Using “is” is more intuitive, more readable, and slightly more efficient, making it by far the better choice.
Using “is” and “as” result in fewer lines of IL and fewer lines of actual C# code by avoiding a lot of extraneous checks and statements, which is evident from the examples above.
Conclusion
There are always plenty of other ways to improve your code and I encourage you to pursue them. The contents of this article, while useful, are only a piece of the puzzle. In this article we looked at a couple of basic ways to clean up your C# code. First we reviewed object disposal and the “IDisposable” interface and how it can be used to clean up unmanaged resources. Second, we talked about the “using” statement, which guarantees proper object disposal. Then we talked about the “is” and “as” operators. “as” provides a safe type cast and “is” provides a safe type check.
The .NET framework provides solid garbage collection and IL optimization, but remember to help out when you can by making sure unmanaged resources are handled and that your objects are disposed of properly. In a managed environment you only have to worry about disposing of types that contain unmanaged resources, or types that are derived from types that implement the “IDisposable” interface. Also be sure to choose operators and statements that allow you to write fewer lines of code and write code that is more readable and more intuitive. Always keep these things in mind when coding in C#.
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