Function to get type of the value entered in a textbox
Here's what you can do:


Since you are not using the out var variable you can remove them altogether and use the discard character which is the underscore to just ignore them _. 
I usually prefer to use {} in ifs but this method is so short and all conditions are so similar that I'd make them one-liners.
Don't call the parameter inputValue but simply value. It might be an input outside but inside this method input has no meaning. The value can come from any source.
Don't return a string but let the user of this method do with the Type whatever he likes or call this method GetTypeName so its purpose is clear.


Example:

public Type GetType(string value)
{
    if (int.TryParse(value, out _)) return typeof(int);
    if (decimal.TryParse(value, out _)) return typeof(decimal);
    if (bool.TryParse(value, out _)) return typeof(bool);

    //If all parse fails, return string type.
    return typeof(string);
}