Force developer to use a particular constructor

Question

I have the following class

  public class UIControl
  {
    public string FName{ get; set; }
    public HtmlInputCheckBox SelectCheckBox { get; set; }
    public bool OverrideSelect { get; set; }

    //Want to throw an ApplicationExceptioh if developer uses this constructor and passes
    //chkSelect = null
    public UIControl(string sFieldName, HtmlInputCheckBox chkSelect)
    {
      this.FName= sFieldName;
      this.SelectCheckBox = chkSelect;
    }

    public UIControl(string sFieldName, HtmlInputCheckBox chkSelect, bool overrideSelect)
      : this(sFieldName, chkSelect)
    {
      OverrideSelect = overrideSelect;
    }
  }

I want to make sure that the developer uses the first constructor only when chkSelect is not null.

I want to do a:

throw new ApplicationException("Dev is using the incorrect constructor");

Answer 1

Whats hard?

public UIControl(string sFieldName, HtmlInputCheckBox chkSelect)
{
    if (chkSelect == null)
    {
        throw new ApplicationException("Dev is using the incorrect constructor");
    }

    this.FName= sFieldName;
    this.SelectCheckBox = chkSelect;
}

Answer 2

Like this?

//Want to throw an ApplicationExceptioh if developer uses this constructor and passes
//chkSelect = null
public UIControl(string sFieldName, HtmlInputCheckBox chkSelect)
{
  if (chkSelect == null)
  {
    throw new ArgumentException("chkSelect cannot be null when using this constructor");
  }
  this.FName= sFieldName;
  this.SelectCheckBox = chkSelect;
}

Answer 3

Why not like this?

public UIControl(string sFieldName, HtmlInputCheckBox chkSelect)
{
    if (chkSelect == null) 
    { 
        throw new ApplicationException("Dev is using the incorrect constructor"); 
    }
    this.FName= sFieldName;
    this.SelectCheckBox = chkSelect;
}

However, it is probably a bad idea to do this via a runtime error. A better idea is to use an overload that doesn't take the chkSelect argument at all, or to make chkSelect non-nullable.

EDIT: I noticed now that the second selector calls the first one, but the only thing the second one does is change the value of the OverrideSelect field. Why not just have one constructor, and set that field? Or why have that field at all? A couple of alternatives:

// This is now the ONLY constructor you need
public UIControl(string sFieldName, HtmlInputCheckBox chkSelect)
{
    OverrideSelect = (chkSelect == null);
    this.FName= sFieldName;
    this.SelectCheckBox = chkSelect;
}


// You could solve it differently by replacing OverrideSelect with this property:
public readonly bool isChkSelectNull {
    get {
        return (this.chkSelect == null);
    }
}

public UIControl(string sFieldName, HtmlInputCheckBox chkSelect)
{
    this.FName= sFieldName;
    this.SelectCheckBox = chkSelect;
}

Answer 4

You can use a private constructor thus:

public UIControl(string sFieldName, HtmlInputCheckBox chkSelect) 
    : this(sFieldName, chkSelect, false, false)
{      
}    

public UIControl(string sFieldName, HtmlInputCheckBox chkSelect, 
     bool overrideSelect)      
    : this(sFieldName, chkSelect, overrideSelect, true)    
{      
}  

private UIControl(string sFieldName, HtmlInputCheckBox chkSelect, 
   bool overrideSelect, bool allowOverride)      
{      
    if ((!allowOverride) && (chkSelect == null)) 
         throw new ArgumentException(...);
    this.FName= sFieldName;      
    this.SelectCheckBox = chkSelect;    
    OverrideSelect = overrideSelect;    
}

There are lots of variants, but as a general rule, have less specific constructors calling more specific ones. For example, the following would also work in your case:

public UIControl(string sFieldName, HtmlInputCheckBox chkSelect)    
    : this(sFieldName, chkSelect, false)
{      
   if (chkSelect == null) throw ...
}    

public UIControl(string sFieldName, HtmlInputCheckBox chkSelect, 
     bool overrideSelect)    
{      
    this.FName= sFieldName;      
    this.SelectCheckBox = chkSelect;    
    this.OverrideSelect = overrideSelect;    
}

Answer 5

Get Spec# or Contracts from .Net 4.0 and make the parameter non-nullable

Answer 6

Based on updated info from the OP:

public class UIControl
{
    //Want to throw an ApplicationException if developer uses this constructor and passes
    //chkSelect = null
    public UIControl(string sFieldName, HtmlInputCheckBox chkSelect)
    {
      if (chkSelect == null)
      {
        throw new ApplicationException("Dev is using the incorrect constructor");
      }
      this.FName= sFieldName;
      this.SelectCheckBox = chkSelect;
    }

    public UIControl(string sFieldName, HtmlInputCheckBox chkSelect, bool overrideSelect)
    {
      this.FName= sFieldName;
      this.SelectCheckBox = chkSelect;  // note: chkSelect MAY be null here.

      OverrideSelect = overrideSelect;
    }
  }

Answer 7

I'm not a big fan of throwing Exceptions for programmer issues. Instead of throwing the Exception, how about Debug.Assert().

Answer 8

Why should you throw an exception if chkSelect is null?

Anyways, to answer what you asked (not what necessarily what you really need), I think you might be doing it backwards. Have the 2 parameter version call the 3 parameter version.

//Want to throw an ApplicationExceptioh if developer uses this constructor and passes
//chkSelect = null
public UIControl(string sFieldName, HtmlInputCheckBox chkSelect)
  : this(sFieldName, chkSelect, chkSelect==null)
{
  if(chkSelect==null) throw new ArgumentNullException("chkSelect");
}

public UIControl(string sFieldName, HtmlInputCheckBox chkSelect, bool overrideSelect)    
{
  this.FName= sFieldName;
  this.SelectCheckBox = chkSelect;
  OverrideSelect = overrideSelect;
}

Answer 9

It's a better practice (or more common practice, since C# doesn't have default parameters until 4.0) to have less specific constructors use the more specific constructors, and not vice versa. You can then utilize a private constructor with a nullable bool. If you are able to rewrite them, try the following:

public UIControl(string sFieldName, HtmlInputCheckBox chkSelect)
  : this(sFieldName, chkSelect, null)
{
}

public UIControl(string sFieldName, HtmlInputCheckBox chkSelect, bool overrideSelect)
  : this(sFieldName, chkSelect, overrideSelect)
{
}

UIControl(string sFieldName, HtmlInputCheckBox chkSelect, bool? overrideSelect)
{
  if (!overrideSelect.HasValue && chkSelect == null)
  {
      throw new ArgumentException("chkSelect");
  }
  FName = sFieldName;
  SelectCheckBox = chkSelect;
  OverrideSelect = overrideSelect ?? false;
}