Using localized exception messages with T4 Templates

A common scenario in many projects is to use a Resource File (.resx) to store exception/validation messages.

An important and useful feature of the .resx files is that you can have one file for each culture that your application supports. For example, you can have one ErrorsMessages.resx as the default to support en-US (English - United States) messages and another with the name ErrorsMessages.pt-BR.resx to support pt-BR (Portuguese - Brazil) messages.

In this tutorial I will show you how to use a T4 Template to read the .resx file and generate a helper class to make very easy handle localized exception messages.

Creating the .resx files

Open your .NET project where you want to have your localized exception messages (probably a class library).

Creating the English (fallback) messages resource file

Add a Resources file called ErrorMessages.resx to the project:

This file will contain the English exception messages for your project. It is also a fallback, if there is no other match for the current UICulture, this one will be used.

Add these lines to the file:

• ProductHasMinWeight: The product ‘{0}’ must weigh at least {1}.
• PropertyIsRequired: The property ‘{0}’ is required.

Creating the Brazilian Portuguese messages resource file

Repeat the same process, but name the file as ErrorMessages-pt-BR.resx. This file will contain your projects Brazilian Portuguese exception messages.

Add these lines to the file:

• ProductHasMinWeight: O produto ‘{0}’ deve pesar pelo menos {1}.

• PropertyIsRequired

  A propriedade ‘{0}’ é obrigatória.

Creating the T4 Template

Create a T4 template file in the same folder where you placed the .resx files, named Errors.tt.

Now, copy the content below to your Errors.tt:

Generating the code

If everything looks good, you should see the following code in the Errors.generated.cs file (expand Errors.tt in Solution Explorer to see it):

As you can see, two methods were generated within the Errors class: PropertyIsRequired and ProductHasMinWeight.

Validating the generated code

If you want to validate that you have followed the tutorial correctly so far, you can create the NUnit project and add this test fixture:

Customizing the T4 Template​

You’ve probably noticed that inside the .tt file there is a line: CHANGE THE VALUES BELOW TO THE VALUES CORRESPONDING TO YOUR PROJECT. In the 3 variables below this line you can customize the name of the resource file, the exception class that will be used (maybe you want to use a different exception or your project has a custom one) and the namespace that the Error class will be in.

Using the generated code

Finally it’s time to use the generated code.

PropertyIsRequired

In this case, it will be generate a InvalidOperationException with the message:

• If UICulture is pt-BR: A propriedade 'First Name' é obrigatória.
• Otherwise: The property 'First Name' is required.

ProductHasMinWeight

In this case, it will be generate a InvalidOperationException with the message:

• If UICulture is pt-BR: O produto 'Amazing Product' deve pesar pelo menos 1KG.
• Otherwise: The product 'Amazing Product' must weigh at least 1KG.

ASP.NET tip

If you want to use this technique in an ASP.NET project remember to enable localization in the Startup.cs or Program.cs file:

Without throwing exceptions

In many projects, throwing exceptions is not possible or desired due to performance and design issues, in these cases you can use the same technique shown in this tutorial, but instead of throwing exceptions, you can adapt T4 to generate an error object business that can go through all layers of your architecture until reaching the UI or the serialization.

Conclusion

T4 is a powerful tool to help create an efficient development flow in the .NET stack. In cases like this tutorial, it drastically reduces the amount of code and improves the overall availability of the codebase.

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