Finding the variable name passed to a function

Question

Let me use the following example to explain my question:

public string ExampleFunction(string Variable) {
    return something;
}

string WhatIsMyName = "         


        

Answer 1

What you want isn't possible directly but you can use Expressions in C# 3.0:

public void ExampleFunction(Expression<Func<string, string>> f) {
    Console.WriteLine((f.Body as MemberExpression).Member.Name);
}

ExampleFunction(x => WhatIsMyName);

Note that this relies on unspecified behaviour and while it does work in Microsoft’s current C# and VB compilers, and in Mono’s C# compiler, there’s no guarantee that this won’t stop working in future versions.

Answer 2

No, but whenever you find yourself doing extremely complex things like this, you might want to re-think your solution. Remember that code should be easier to read than it was to write.

Answer 3

Thanks for all the responses. I guess I'll just have to go with what I'm doing now.

For those who wanted to know why I asked the above question. I have the following function:

string sMessages(ArrayList aMessages, String sType) {
    string sReturn = String.Empty;
    if (aMessages.Count > 0) {
        sReturn += "<p class=\"" + sType + "\">";
        for (int i = 0; i < aMessages.Count; i++) {
            sReturn += aMessages[i] + "<br />";
        }
        sReturn += "</p>";
    }
    return sReturn;
}

I send it an array of error messages and a css class which is then returned as a string for a webpage.

Every time I call this function, I have to define sType. Something like:

output += sMessages(aErrors, "errors");

As you can see, my variables is called aErrors and my css class is called errors. I was hoping my cold could figure out what class to use based on the variable name I sent it.

Again, thanks for all the responses.

Answer 4

Do this

var myVariable = 123;
myVariable.Named(() => myVariable);
var name = myVariable.Name();
// use name how you like

or naming in code by hand

var myVariable = 123.Named("my variable");
var name = myVariable.Name();

using this class

public static class ObjectInstanceExtensions
{
    private static Dictionary<object, string> namedInstances = new Dictionary<object, string>();

    public static void Named<T>(this T instance, Expression<Func<T>> expressionContainingOnlyYourInstance)
    {
        var name = ((MemberExpression)expressionContainingOnlyYourInstance.Body).Member.Name;
        instance.Named(name);            
    }

    public static T Named<T>(this T instance, string named)
    {
        if (namedInstances.ContainsKey(instance)) namedInstances[instance] = named;
        else namedInstances.Add(instance, named);
        return instance;
    }        

    public static string Name<T>(this T instance)
    {
        if (namedInstances.ContainsKey(instance)) return namedInstances[instance];
        throw new NotImplementedException("object has not been named");
    }        
}

Code tested and most elegant I can come up with.

Answer 5

No. A reference to your string variable gets passed to the funcion--there isn't any inherent metadeta about it included. Even reflection wouldn't get you out of the woods here--working backwards from a single reference type doesn't get you enough info to do what you need to do.

Better go back to the drawing board on this one!

rp

Answer 6

Well had a bit of look. of course you can't use any Type information. Also, the name of a local variable is not available at runtime because their names are not compiled into the assembly's metadata.