T4 Text Template Utility Methods

Статья
02/04/2013

There are several methods that are always available to you when you write code in a Visual Studio text template. These methods are defined in TextTransformation.

Tip

You can also use other methods and services provided by the host environment in a regular (not preprocessed) text template. For example, you can resolve file paths, log errors, and get services provided by Visual Studio and any loaded packages. For more information, see Accessing Visual Studio or other Hosts from a T4 Text Template.

Write methods

You can use the Write() and WriteLine() methods to append text inside a standard code block, instead of using an expression code block. The following two code blocks are functionally equivalent.

Code block with an expression block

<#
int i = 10;
while (i-- > 0)
    { #>
        <#= i #>
    <# }
#>

Code block using WriteLine()

<# 
    int i = 10;
    while (i-- > 0)
    { 
        WriteLine((i.ToString()));
    }
#>

You may find it helpful to use one of these utility methods instead of an expression block inside a long code block with nested control structures.

The Write() and WriteLine() methods have two overloads, one that takes a single string parameter and one that takes a composite format string plus an array of objects to include in the string (like the Console.WriteLine() method). The following two uses of WriteLine() are functionally equivalent:

<#
    string msg = "Say: {0}, {1}, {2}";
    string s1 = "hello";
    string s2 = "goodbye";
    string s3 = "farewell";
    
    WriteLine(msg, s1, s2, s3);
    WriteLine("Say: hello, goodbye, farewell");
#>

Indentation methods

You can use indentation methods to format the output of your text template. The TextTransformation class has a CurrentIndent string property that shows the current indentation in the text template and an indentLengths field that is a list of the indentations that have been added. You can add an indentation with the PushIndent() method and subtract an indentation with the PopIndent() method. If you want to remove all indentations, use the ClearIndent() method. The following code block shows the use of these methods:

<#
    WriteLine(CurrentIndent + "Hello");
    PushIndent("    ");
    WriteLine(CurrentIndent + "Hello");
    PushIndent("    ");
    WriteLine(CurrentIndent + "Hello");
    ClearIndent();
    WriteLine(CurrentIndent + "Hello");
    PushIndent("    ");
    WriteLine(CurrentIndent + "Hello");
#>

This code block produces the following output:

Hello
        Hello
                Hello
Hello
        Hello

Error and warning methods

You can use error and warning utility methods to add messages to the Visual Studio Error List. For example, the following code will add an error message to the Error List.

<#
  try
  {
    string str = null;
    Write(str.Length.ToString());
  }
  catch (Exception e)
  {
    Error(e.Message);
  }
#>

Access to Host and Service Provider

The property this.Host can provide access to properties exposed by the host that is executing the template. To use this.Host, you must set hostspecific attribute in the <@template#> directive:

<#@template ... hostspecific="true" #>

The type of this.Host depends on the type of host in which the template is executing. In a template that is running in Visual Studio, you can cast this.Host to IServiceProvider to gain access to services such as the IDE. For example:

EnvDTE.DTE dte = (EnvDTE.DTE) ((IServiceProvider) this.Host)
                       .GetService(typeof(EnvDTE.DTE));

Using a different set of utility methods

As part of the text generation process, your template file is transformed into a class, which is always named GeneratedTextTransformation and inherits from TextTransformation. If you want to use a different set of methods instead, you can write your own class and specify it in the template directive. Your class must inherit from TextTransformation.

<#@ template inherits="MyUtilityClass" #>

Use the assembly directive to reference the assembly where the compiled class can be found.

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