C# Passing properties by reference

Question

Hey guys I'm trying to do do the following.

GetString(
    inputString,
    ref Client.WorkPhone)

...

private void GetString(string in, ref string out)
{
    if (!string.IsNullOrEmpty(in))
    {
        out = in;
    }
}

This is giving me a compile error, I think its pretty clear what I'm trying to achieve basically I want GetString to copy the contents of an input string "Input String" to the WorkPhone property of Client.

Is it possible to pass a property by reference?

Thanks in advance.

Answer 1

cannot pass properties by reference

Answer 2

This is not possible. You could say

Client.WorkPhone = GetString(inputString, Client.WorkPhone);

where WorkPhone is a writeable string property and the definition of GetString is changed to

private string GetString(string in, string current) { 
    if (!string.IsNullOrEmpty(in)) {
        return in;
    }
    return current;
}

This will have the same semantics that you seem to be trying for.

This isn't possible because a property is really a pair of methods in disguise. Each property makes available getters and setters that are accessible via field-like syntax. When you attempt to call GetString as you've proposed, what you're passing in is a value and not a variable. The value that you are passing in is that returned from the getter get_WorkPhone.

Answer 3

This is covered in section 7.4.1 of the C# language spec. Only a variable-reference can be passed as a ref or out parameter in an argument list. A property does not qualify as a variable reference and hence cannot be used.

Answer 4

Properties cannot be passed by reference. Here are a few ways you could work around this limitation.

1. Return Value

string GetString(string input, string output)
{
    if (!string.IsNullOrEmpty(input))
    {
        return input;
    }
    return output;
}

void Main()
{
    var person = new Person();
    person.Name = GetString("test", person.Name);
    Debug.Assert(person.Name == "test");
}

2. Delegates

void GetString(string input, Func<string> getOutput, Action<string> setOutput)
{
    if (!string.IsNullOrEmpty(input))
    {
        setOutput(input);
    }
}

void Main()
{
    var person = new Person();
    GetString("test", () => person.Name, value => person.Name = value);
    Debug.Assert(person.Name == "test");
}

3. Linq Expressions

void GetString<T>(string input, T outObj, Expression<Func<T, string>> outExpr)
{
    if (!string.IsNullOrEmpty(input))
    {
        var expr = (MemberExpression) outExpr.Body;
        var prop = (PropertyInfo) expr.Member;
        prop.SetValue(outObj, input, null);
    }
}

void Main()
{
    var person = new Person();
    GetString("test", person, x => x.Name);
    Debug.Assert(person.Name == "test");
}

Answer 5

What you could try to do is create an object to hold the property value. That way you could pass the object and still have access to the property inside.