is there any way to make this code shorter?

Question

//list the controls from the  main form
foreach (Control c in Controls)
{
    if (c is ComboBox)
    {
        ((ComboBox)c).SelectedIndex = -1;
    }
    else if (c is TextBox)
    {
        ((TextBox)c).Text = "";
    }
    else if (c is CheckBox)
    {
        ((CheckBox)c).Checked = false;
    }
    //etc. with FIFTY different types to check against
}

Answer 1

One way is to add three overloaded methods for specific types, cast to dynamic, and make calls like this:

foreach (dynamic c in Controls) {
    ClearOut(c);
}
...
private static void ClearOut(ComboBox c) {
    c.SelectedIndex = -1;
}
private static void ClearOut(TextBox t) {
    t.Text = string.Empty;
}
private static void ClearOut(CheckBox c) {
    c.Checked = false;
}

Since c is dynamic, C# would postpone binding of the ClearOut method to runtime, giving you a clean-looking code. The downside of this approach is that C# is unable to tell you at compile-time if one of the overloads is missing.

Answer 2

Use this method to set the properties of your controls:

public void Set(object obj, string property, object value)
{
    //use reflection to get the PropertyInfo of the property you want to set
    //if the property is not found, GetProperty() returns null
    var propertyInfo = obj.GetType().GetProperty(property);
    //use the C# 6 ?. operator to only execute SetValue() if propertyInfo is not null
    propertyInfo?.SetValue(obj, value);
}

Call it like this:

foreach (Control c in Controls)
{
    Set(c, "SelectedIndex", -1);
    Set(c, "Text", "");
    Set(c, "Checked", false);
}

Answer 3

You can create a lookup from each supported type to an action that clears controls of that type, you can then add handlers to that lookup for each supported type:

public class ControlClearer
{
    private static Dictionary<Type, Action<Control>> lookup = new Dictionary<Type, Action<Control>>();

    static ControlClearer()
    {
        AddMapping((TextBox control) => control.Text = "");
        AddMapping((ComboBox control) => control.SelectedIndex = -1);
        AddMapping((CheckBox control) => control.Checked = false);
    }

    private static void AddMapping<T>(Action<T> clearAction)
        where T : Control
    {
        lookup[typeof(T)] = control => clearAction((T)(object)control);
    }

    public static void Clear<T>(T control)
        where T : Control
    {
        //todo support case where T isn't in the dictionary
        lookup[typeof(T)](control);
    }

    public static void Clear(Control control)
    {
        //todo support case where the type isn't in the dictionary
        lookup[control.GetType()](control);
    }
}