#include directive in C#

Question

Is there a replacement? If there is, how would the directive look for a file named "class.cs"? I just want to split the code into a file for each class.

Answer 1

The idiomatic way to achieve metaprogramming in C# (beyond Generics) is with T4 templates - Visual Studio and MSBuild supports T4 built-in, however VS does not come with T4 syntax coloring - you'll need a third-party add-in for that.

In order to demonstrate T4's include functionality, I'll use the scenario of wanting to add an == operator overload to multiple classes simultaneously without using inheritance.

For comparison, in C++ it would be like this:

OperatorEquals.inc

bool operator==(const TYPE* lhs, const TYPE* rhs) { if( lhs == nullptr && rhs != nullptr ) return false; return lhs.Equals(rhs); }

Code.h

class Foo {
public:
#define TYPE Foo
#include "OperatorEquals.inc"
}

class Bar {
public:
#define TYPE Bar
#include "OperatorEquals.inc"
}

In C#, you would do this:

1. Use partial classes so that all of your non-metaprogramming logic (i.e. normal C# code) is in a file, e.g. Foo.cs and Bar.cs
2. Create a new T4 template in your project, change the output file extension to .cs
3. Create a second partial class definition of the same type within that T4 (*.tt) file, though you won't have C# syntax highlighting.
4. Define the included file:

Operators.inc.cs.t4

public static operator==(<#= typeName #> x, <#= typeName #> y) {
    if( x == null && y != null ) return false;
    return x.Equals( y );
}

5. Add it to your T4 template:

Metaprogramming.tt

<#@ template debug="false" hostspecific="false" language="C#" #>
<#@ import namespace="System" #>
<#@ output extension=".cs" #>
<# String typeName = null; #>

public partial class Foo {

    <# typeName = "Foo"; #>
    <#@ include file="Operators.inc.cs.t4" #>
}

public partial class Bar {

    <# typeName = "Bar"; #>      
    <#@ include file="Operators.inc.cs.t4" #>
}

Whenever you "Save" the .tt file (even if you make no changes) VS will regenerate the output .cs file which will look like this:

public partial class Foo {

    public static operator==(Foo x, Foo y) {
       if( x == null && y != null ) return false;
       return x.Equals( y );
   }
}

public partial class Bar {

   public static operator==(Bar x, Bar y) {
       if( x == null && y != null ) return false;
       return x.Equals( y );
   }
}

Note that this scenario is contrived - if you really did want to add the operator== (and all the others: IEquatable<T>, operator!=, IComparable<T>, etc) then you would probably use a T4 render function instead of an include, because that makes parameterization more straightforward and keeps everything self-contained in a single file:

T4RenderFunction.tt

<#@ template debug="false" hostspecific="false" language="C#" #>
<#@ import namespace="System" #>
<#@ output extension=".cs" #>
<# String typeName = null; #>

public partial class Foo {

    <# RenderBoilerplateOperators("Foo"); #>
}

public partial class Bar {

    <# RenderBoilerplateOperators("Bar"); #>
}

<#+
// Functions are declared at the bottom
void RenderBoilerplateOperators(String typeName) {
#>
    public static operator==(<#= typeName #> lhs, <#= typeName #> rhs) {
        return <#= typeName #>.Equals( lhs, rhs );
    }

    public override Boolean Equals(<#= typeName #> other) {
        return <#= typeName #>.Equals( this, other );
    }

    public static Boolean Equals(<#= typeName #> lhs, <#= typeName #> rhs) {
        // T4 can use VS DTE to enumerate members of `typeName`, but you're probably better-off implementing this method manually
    }

    public static operator!=(<#= typeName #> lhs, <#= typeName #> rhs) {
        return !<#= typeName #>.Equals( lhs, rhs );
    }

    // and so on...
<#
} // void RenderBoilerplateOperators
#>

Answer 2

No, there is no replacement for an #include statement. C# is an object-oriented language where code is organised into classes. You can use code from one class in another class depending on its visibility, and you can split the code from a single class across multiple source files using partial classes. These are essentially the way you use code from one "file" in another. But it's not the same thing at all.