Auto-Generating EventArgs classes with T4

T4 is a code generator built into Visual Studio. In this post, I will present a T4 template file which I've used to help auto-generate Event Arguments classes in C#.

Normally, event arguments classes always follow the same structure. You typically have a set of public properties in a class which extends EventArgs and a constructor which takes in the same number of formal arguments.

Here's a typical event arguments class:

public class OperationCompletedEventArgs : EventArgs
{
    public bool Result { get; private set; }

    public OperationCompletedEventArgs(bool result)
    {
        Result = result;
    }
}

Given the repetitive nature of how typical event arguments classes are structured, they make a perfect fit for an example of code generation.

---

The intent of my template file for generating event arguments classes is to be able to define a declaration of what classes I want to be generated and then let the template generate those classes for me.

The declaration part of the template looks something like this:

var collection = new List<EventArgumentsRepresentation>
                 {
                     new EventArgumentsRepresentation("OperationCompleted")
                     {
                         Members = new Dictionary<string, string> { 
                             {"Result", "bool"}
                         }
                     },
                     new EventArgumentsRepresentation("GenericAndPartial")
                         {
                             AccessModifier = "internal",
                             IsPartial = true,
                             GenericTypes = new List<string>() { "T", "TK"},
                             Members = new Dictionary<string, string> { 
                                 {"GenericMember1", "T"},
                                 {"GenericMember2", "TK"}
                             }
                         }
                 };

Given the above structure, the template would then generate the following two classes for me:

public class OperationCompletedEventArgs : EventArgs
{
    public bool Result { get; private set; }

    public OperationCompletedEventArgs(bool result) 
    {
        Result = result;
    }
}

internal partial class GenericAndPartialEventArgs<T, TK> : EventArgs
{
    public T GenericMember1 { get; private set; }
    public TK GenericMember2 { get; private set; }

    public GenericAndPartialEventArgs(T genericMember1, TK genericMember2) 
    {
        GenericMember1 = genericMember1;
        GenericMember2 = genericMember2;
    }
}

The next I would want a new event arguments class, I would just add a new instance of EventArgumentsRepresentation to the list and let the template do the work.

---

And now, for the entire template file. If you want to use it for your own projects, create a new "Text Template" (.tt file) in Visual Studio and copy the below template into it. Once you then save the template file, the classes will be generated into a .cs file.

<#@ template debug="false" hostspecific="false" language="C#" #>
<#@ assembly name="System.Core" #>
<#@ import namespace="System.Linq" #>
<#@ import namespace="System.Collections.Generic" #>
<#@ output extension=".cs" #>

<#
var namespaceName = "EventArgumentsGenerationDemo";
var usings = new List<string> {
    "System",
};

var collection = new List<EventArgumentsRepresentation>
                 {
                     new EventArgumentsRepresentation("OperationCompleted")
                     {
                         Members = new Dictionary<string, string> { 
                             {"Result", "bool"},
                         },
                     },
                     new EventArgumentsRepresentation("GenericAndPartial")
                         {
                             AccessModifier = "internal",
                             IsPartial = true,
                             GenericTypes = new List<string>() { "T", "TK"},
                             Members = new Dictionary<string, string> { 
                                 {"GenericMember1", "T"},
                                 {"GenericMember2", "TK"},
                             },
                         },
                     
                 };
#>
namespace <#=namespaceName#>
{
<#
foreach (var usingDirective in usings) {
#>
    using <#=usingDirective#>;
<#
}
#>

    /*
     * This file is autogenerated.  
     *
     * If you want to edit anything from here, use the EventArgumentsClasses.tt file.
     */

<#
foreach (var eventArgsRepresentation in collection) {
#>
    <#=eventArgsRepresentation.ClassSignature#>
    {
<#
foreach (var member in eventArgsRepresentation.Members) {
#>
        public <#=member.Value#> <#=EventArgumentsRepresentation.ToPascalCase(member.Key)#> { get; private set; }
<#
}
#>

        public <#=eventArgsRepresentation.ClassName#>(<#=eventArgsRepresentation.GetConstructorArguments()#>) 
        {
<#
foreach (var member in eventArgsRepresentation.Members) {
#>
            <#=EventArgumentsRepresentation.ToPascalCase(member.Key)#> = <#=EventArgumentsRepresentation.ToCamelCase(member.Key)#>;
<#
}
#>
        }
    }

<#
}
#>
}

<#+
internal class EventArgumentsRepresentation
{
    public string ClassName { get; private set; }

    public string AccessModifier { get; set; }
    public bool IsPartial { get; set; }
    public List<string> GenericTypes {get; set;}
    public Dictionary<string, string> Members { get; set; }
    public string ClassSignature {
        get {
            return String.Format("{0} {1}class {2}{3} : EventArgs", AccessModifier, IsPartial ? "partial " : "", ClassName, GetGenericTypesSignature());
        }
    }

    public EventArgumentsRepresentation(string name)
    {
        ClassName = String.Format("{0}EventArgs", ToPascalCase(name));
        AccessModifier = "public";
        GenericTypes = new List<string>();
        Members = new Dictionary<string, string>();
    }

    public string GetGenericTypesSignature() {
        if (GenericTypes.Count < 1) {
            return "";
        }

        return String.Format("<{0}>", string.Join(", ", GenericTypes.ToArray()));
    }

    public string GetConstructorArguments() {
        if (Members == null || Members.Count < 1) {
            return "";
        }

        return String.Join(", ", Members.Select(k => String.Format("{0} {1}", k.Value, ToCamelCase(k.Key))).ToArray());
    }

    public static string ToPascalCase(string text) {
        return char.ToUpper(text[0]) + text.Substring(1);
    }

    public static string ToCamelCase(string text) {
        return char.ToLower(text[0]) + text.Substring(1);
    }
}
#>