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September 18, 2002 Catching, Manipulating, and Throwing Exceptions  Tips: September 2002
Yehuda Shiran, Ph.D.
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Yesterday, we showed you how to write a JScript .NET function that handles a null parameter with two nested try...catch blocks. The function TryCatchDemo() throws string exceptions from one block to the other. One exception is thrown from the inner try block to the inner catch block. Another exception is thrown from the inner catch block to the outer catch block. We call the function TryCatchDemo() twice: once with a null argument, and once with an array:
TryCatchDemo() and the two calls shown above. The window also shows how to compile the code (throw2.js), and the response of the function to these two calls. When we pass a null value, the response is "x equals null and handled in inner catch block". When we pass a real array, the response is "x does not equal null, error handled in outer catch block". Notice that each final exception string is built by the exception handling block that catches the exception and throws it farther to the outer block. If x is null, the inner try block throws "x equals null". The inner catch block catches this exception and, just before returning, adds a few words to the exception string (" and handled in inner catch block"). If x is not null, the inner try block throws "x does not equal null". The inner catch block catches this exception and throws it up again. The outer catch block catches this exception and, just before returning, adds a few words to the exception string (" , error handled in outer catch block"). Here is the Command Prompt window:
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