# Writing extension functions for .NET

On the .NET platform extension functions may be implemented in any .NET language (the examples here assume C#).

Queries and stylesheets running on the .NET platform may also call Java extension functions, provided the Java class is part of the standard library implemented in the GNU Classpath DLL, or in Saxon itself. For calling conventions, see Writing extension functions in Java. If you want to write your own extensions in Java, you will need to compile them to .NET assemblies using IKVMC.

An extension function is invoked using a name such as `prefix:localname()`. The prefix must be the prefix associated with a namespace declaration that is in scope. The namespace URI is used to identify a .NET class, and the local name is used to identify a method, property, or constructor within the class.

The command line option -TJ is useful for debugging the loading of .NET extensions. It gives detailed information about the methods that are examined for a possible match.

The basic form of the namespace URI is "clitype:" followed by the fully-qualified type name (for example `xmlns:env="clitype:System.Environment"`). This form works for system classes and classes in a loaded assembly. If an assembly needs to be loaded, extra information can be given in the form of query parameters. For example `xmlns:env="type:Acme.Payroll.Employee?asm=payroll;version=4.12.0.0"`.

The parameters that are recognized are:

Keyword | Value
asm | The simple name of the assembly
ver | The version number, up to four integers separated by periods
loc | The culture (locale), for example "en-US"
sn | The public key token of the assembly's strong name, as 16 hex digits
from | The location of the assembly, as a URI
partialname | The partial name of the assembly (as supplied to `Assembly.LoadWithPartialName()`).

If the `from` keyword is present, the other parameters are ignored. The value of `from` must be a URI: if it is relative, it is relative to the current directory from which the application is launched.

If the `partialName` keyword is present, the assembly is loaded (if possible) using `Assembly.LoadWithPartialName()` and the other parameters are ignored.

Note that loading an assembly by simple name works only in limited cases: for example, if the assembly is already loaded, or if it is in the same directory as the entry point of the application. This option works well if the extension function is in a class contained in the same assembly as the application that fired off the query or transformation.

If the assembly is a library DLL in the global assembly cache, use the `gacutil /l` command to list the assemblies present in the GAC, and to extract the required version, culture, and strong name attributes. For example, suppose you want to call a static method `disappear()` in class `Conjurer` in namespace `Magic.Circle`, and this class is contained in an assembly `Magic` listed as:

`Magic, Version=7.2.2200.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a4b, Custom=null`

Then the URI you would use to identify this class is `clitype:Magic.Circle.Conjurer?asm=Magic;ver=7.2.2200.0;sn=b03f5f7f11d50a4b`, and an actual call of the function might take the form:

    <xsl:value-of select="m:disappear()"
         xmlns:m="clitype:Magic.Circle.Conjurer?asm=Magic;ver=7.2.2200.0;sn=b03f5f7f11d50a4b"/>

The rest of this section considers how a .NET method, property, or constructor is identified. This decision (called binding) is always made at the time the XPath expression is compiled.

There are three cases to consider: static methods, constructors, and instance-level methods. In addition, a public property in a class is treated as if it were a zero-argument method, so static properties can be accessed in the same way as static methods, and instance-level properties in the same way as instance-level methods. (Note that the property name is used directly: it is not prefixed by "get".)

- Calling Static Methods in a .NET Class

- Calling .NET Constructors

- Calling .NET Instance-Level Methods

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