Throw

The throw-keyword in C# generates (or translates) exceptions. By using a throw-statement inside a catch block, we can change the resulting exception.

Notes, throw

We can throw a new exception. The throw-statement is versatile—throw is a statement, but also a special kind of expression.

Example

We first look at 3 methods A, B, and C that use the throw-statement in different ways. Method A uses a throw0statement with no argument.

Info The code in method A can be thought of as a rethrow—it throws the same exception already being handled.

Next Method B throws a named exception variable. This is not a rethrow—it throws the same exception but changes the stack trace.

Also We can collect information about the exception if needed. Method C creates a new exception.

Tip We can use a throw-statement in this way to implement custom error conditions.

using System;

class Program
{
    static void Main()
    {
        // Call method A, method B, and method C.
        // ... Comment out the first 1-2 method invocations.
        try
        {
            A();
            B();
            C(null);
        }
        catch (Exception ex)
        {
            Console.WriteLine(ex);
        }
    }

    static void A()
    {
        // Rethrow syntax.
        try
        {
            int value = 1 / int.Parse("0");
        }
        catch
        {
            throw;
        }
    }

    static void B()
    {
        // Filtering exception types.
        try
        {
            int value = 1 / int.Parse("0");
        }
        catch (DivideByZeroException ex)
        {
            throw ex;
        }
    }

    static void C(string value)
    {
        // Generate new exception.
        if (value == null)
        {
            throw new ArgumentNullException("value");
        }
    }
}System.DivideByZeroException: Attempted to divide by zero.
System.DivideByZeroException: Attempted to divide by zero.
System.ArgumentNullException: Value cannot be null.
Parameter name: value

Rethrow

Next we examine rethrows in the C# language. A rethrow must use a throw-statement with no argument. If you use throw ex, then the TargetSite and StackTrace are changed.

Detail In this program, X() uses a rethrow statement. And Y() uses a throw ex statement.

Result We see in the results that when a rethrow is used, the exception TargetSite is in StringToNumber—an internal method of int.Parse.

But When a throw ex is used, as in Y(), the exception's TargetSite was modified to the current method Y().

using System;

class Program
{
    static void Main()
    {
        try
        {
            X();
        }
        catch (Exception ex)
        {
            Console.WriteLine(ex.TargetSite);
        }

        try
        {
            Y();
        }
        catch (Exception ex)
        {
            Console.WriteLine(ex.TargetSite);
        }
    }

    static void X()
    {
        try
        {
            int.Parse("?");
        }
        catch (Exception)
        {
            throw; // [Rethrow construct]
        }
    }

    static void Y()
    {
        try
        {
            int.Parse("?");
        }
        catch (Exception ex)
        {
            throw ex; // [Throw captured ex variable]
        }
    }
}Void StringToNumber(System.String, ...)
Void Y()

Throw expression

Throw is usually used as a statement. But in some places, it can be used as an expression. We can use throw as part of a null-coalescing statement.

Here Consider the Test() method. If its argument is null, a new ArgumentException is thrown as part of the null-coalescing statement.

using System;

class Program
{
    static void Test(string argument)
    {
        // Use null coalescing operator with throw expression.
        // ... If argument is null, throw.
        var value = argument ?? throw new ArgumentException("Bad argument");
        Console.WriteLine("HERE");
    }

    static void Main()
    {
        Test("bird");
        Test(null);
    }
}HERE

Unhandled Exception: System.ArgumentException: Bad argument
   at Program.Test(String argument) in c:\users\...Program.cs:line 9
   at Program.Main() in c:\users\...Program.cs:line 16

Throw, ternary expression

A throw-expression can only be used in certain places in programs—not everywhere. We can use a throw-expression as part of a ternary statement.

Here The string's length is tested. If it has fewer than 10 characters, an Exception is thrown as part of the ternary.

using System;

class Program
{
    static void Main()
    {
        string test = "bird";
        // Throw can be used in ternary.
        char tenth = test.Length >= 10 ?
            test[9] :
            throw new Exception("Not ten characters");
        // Not reached.
        Console.WriteLine(tenth);
    }
}Unhandled Exception: System.Exception: Not ten characters
   at Program.Main() in c:\users\...Program.cs:line 9

A review

The exception handling mechanism exploits an alternative control pathway. The throw-statement provides the ability to rethrow an exception or generate a new one.

We can use throw as a statement, or an expression in some places in our programs. Though the syntax is confusing at first, it becomes easier to use with familiarity.