This small tip about how to show the negative currency value in your application. For Example if you have currency value like -1234 and you want to display it like -$1,234 according to you culture.

Problem
In C# to one of the way to format currency value easily is use ToString("c") with value will do work for you. For example check below code

//format -1234 as currency..
Console.WriteLine((-1234).ToString("c"));

Output
This will display value in output like this ($1,234.00). But the actual problem with the formatting is its not displaying - sign with currency value.
Solution
So to come out of this You need to create a custom NumberFormatInfo from your current locale. Then you can specify it's CurrencyNegativePattern, for example:

CultureInfo currentCulture = Thread.CurrentThread.CurrentCulture;
CultureInfo newCulture = new CultureInfo(currentCulture.Name);
newCulture.NumberFormat.CurrencyNegativePattern = 1;
Thread.CurrentThread.CurrentCulture = newCulture;
Console.WriteLine((-1234).ToString("c"));

Output
Now output of the above code is -$1,234.00. That's what actually needed display negative sign for negative currency value.
So in above code actual trick done by NumberFormatInfo.CurrencyNegativePattern Property, which is set to 1 which associated with pattern -$n. There number of different pattern supported which you can check on MSDN and assign value to property according to your need. This will not affect positive value of currency i.e. if you pas 1234 as value its dispaly as $1,234 only. And for this this example my current culture is "en-US" .

Conclusion
I Hope you all like this simple and easy example of formatting negative currency. you can explore more on MSDN on reference provided at end of post.
NumberFormatInfo Class - http://msdn.microsoft.com/en-us/library/system.globalization.numberformatinfo.aspx
NumberFormatInfo.CurrencyNegativePattern Property - http://msdn.microsoft.com/en-us/library/system.globalization.numberformatinfo.currencynegativepattern.aspx