.NET Tips and Tricks

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Checking If You Can Use a Class in C#

The is keyword lets you check if a variable is pointing to an object of a particular class (or a class that inherits from some class). For example, the code in this if block only executes if CustomerVariable is pointing at an object of type Customer (or some class that inherits from Customer):

if (CustomerVariable is Customer)
{
  ...code to execute...
}

It works with interfaces, too:
if (CustomerVariable is ICustomer)
{
  ...code to execute ...
}

You can do your test and get a free cast from a variable by using the as keyword. If the cast fails, no exception is raised, but your destination variable is set to null, telling you that the variable isn't compatible with the object. The following code not only does what the previous code did, but also casts whatever object PremiumCustomerVariable is pointing to into CustomerVariable (if CustomerVariable and the object are compatible, of course):

CustomerVariable = PremiumCustomerVariable as Customer;
if (CustomerVariable != null)
{
  ...code to execute if PremiumCustomerVariable could be cast as a Customer...
}

If you want to do the same thing using the variable's class (rather than using some object as the previous examples did) then you want to use IsAssignableFrom:

if (typeof(Customer).IsAssignableFrom(typeof(PremiumCustomer)))
{
  CustomerVariable = PremiumCustomerVariable;
}

This also works with interfaces:

if (typeof(ICustomer).IsAssignableFrom(typeof(PremiumCustomer)))
{
  ICustomerVariable = PremiumVariable;
}

What other ways do you use to check? Share in the comments section.

Posted by Peter Vogel on 08/04/2015 at 2:20 PM


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