Question

I’m still trying to wrap my head around delegate functions and extension methods. I have created an extension method for DropDownList. I’d like to pass the function to be called in my extension method but I’m getting an error Argument type 'IOrderedEnumerable<KeyValuePair<string,string>>' is not assignable to parameter type 'System.Func<IOrderedEnumerable<KeyValuePair<string,string>>>'

public static class DropDownListExtensions {
    public static void populateDropDownList(this DropDownList source, Func<IOrderedEnumerable<KeyValuePair<string, string>>> delegateAction) {
        source.DataValueField = "Key";
        source.DataTextField = "Value";
        source.DataSource = delegateAction;
        source.DataBind();
    }
}

being called like so…

myDropDownList.populateDropDownList(getDropDownDataSource());

getDropDownDataSource signature…

protected IOrderedEnumerable<KeyValuePair<string,string>> getDropDownDataSource() {
    StateInfoXmlDocument stateInfoXmlDocument = new StateInfoXmlDocument();
    string schoolTypeXmlPath = string.Format(STATE_AND_SCHOOL_TYPE_XML_PATH, StateOfInterest, SchoolType);
    var nodes = new List<XmlNode>(stateInfoXmlDocument.SelectNodes(schoolTypeXmlPath).Cast<XmlNode>());
    return nodes.Distinct().Select(x => new KeyValuePair<string, string>(x.Attributes["area"].Value, x.Attributes["area"].Value)).OrderBy(x => x.Key);
}

Answer 1

You should remove the () after getDropDownDataSource when calling:

myDropDownList.populateDropDownList(getDropDownDataSource);

EDIT: Method groups can be implicitly converted to delegates with a compatible signature. In this case getDropDownDataSource matches the signature of Func<IOrderedEnumerable<KeyValuePair<string,string>>> so the compiler applies the conversion for you, effectively doing

Func<IOrderedEnumerable<KeyValuePair<string,string>>> func = getDropDownDataSource;
myDropDownList.populateDropDownList(func);