Conditional Expression Testing for Nulls

**Jon Skeet [C# MVP]** · Feb 24 '06, 07:55 PM

Re: Conditional Expression Testing for Nulls

a <a@discussions. microsoft.com> wrote:[color=blue]
> I'm having trouble testing a custom object. I've tried many different
> approaches. One is shown below.
>
> The XML below shows the state of the object and I'm trying to test for that
> state, ie there are NO classes in the Classes collection (Classes) of the
> custom object.
>
> The conditional expression returns the error shown below it...[/color]

<snip>

How about

if (Profile.Teache rs[0].Classes.Count= =0)

It's hard to say exactly whether that'll work or not without knowing
what the type of the property "Classes" is, but it's worth a try...

--
Jon Skeet - <skeet@pobox.co m>
http://www.pobox.com/~skeet Blog: http://www.msmvps.com/jon.skeet
If replying to the group, please do not mail me too

a · Feb 24 '06, 09:05 PM

Re: Conditional Expression Testing for Nulls

Jon:

I tried that, as well as everything else, but no go.

The Count method does not exist on the Teahcers object, just on the Teacer
object.

I wondered if there was a problem with the indexer because I'm using this
inside a dropdownlist, but I don't think that is the problem. It may have
more to do with the way that I structured the custom object. Since this is
my first custom object, I definitely don't know what I'm doing.

I'll include the custom object. It's purpose is to has Teachers who have
Classes that in turn have Students.

Any other ideas?

Thanks
---------------------------------------------------------------------------

using System;
using System.Collecti ons;
using System.Xml.Seri alization;//[XmlIgnore] // XmlIgnore is need for
proper nesting of the xml output

namespace Teachers1
{
#region Student
Object+++++++++ +++++++++++++++ +++++++++++++++ +++++++++++++++ ++

[Serializable()]
public class Student
{
#region Private members (Fields) +++++++++++++++ +++++

int id;
string name;

#endregion -------------------------------------------

#region Public Accessors (Properties) ++++++++++++++

[XmlAttribute]
public int Id
{
get { return this.id; }
set { this.id = value; }
}

[XmlAttribute]
public string Name
{
get { return this.name; }
set { this.name = value; }
}

#endregion -------------------------------------------

#region Constructors +++++++++++++++ +++++++++++++++ +++

// Empty constructor
public Student()
{ }

// Constructor 2
public Student(int Id, string Name)
{
this.id = Id;
this.name = Name;
}

#endregion -------------------------------------------
}

#endregion

[Serializable()]
public class Students : CollectionBase
{
#region Indexer +++++++++++++++ +++++++++++++++ +++

public Student this[int index]
{
set
{
List[index] = value;
}
get
{
return (Student)List[index];
}
}

#endregion ------------------------------------

#region Public Methods +++++++++++++++ +++++++++++

public int Add(Student value)
{
return List.Add(value) ;
}
public int IndexOf(Student value)
{
return List.IndexOf(va lue);
}
public void Insert(int index, Student value)
{
List.Insert(ind ex, value);
}
public void Remove(Student value)
{
List.Remove(val ue);
}
public bool Contains(Studen t value)
{
return List.Contains(v alue);
}

#endregion -------------------------------------------

#region Constructors +++++++++++++++ +++++++++++++++ +++

public Students()
{ }

#endregion -------------------------------------------
}

#region Class
Object+++++++++ +++++++++++++++ +++++++++++++++ +++++++++++++++ ++++

[Serializable()]
public class Class
{
#region Private members (Fields) +++++++++++++++ +++++

int id;
string name;
Teachers1.Stude nts students = new Teachers1.Stude nts();

#endregion -------------------------------------------

#region Public Accessors (Properties) ++++++++++++++

[XmlAttribute]
public int Id
{
get { return id; }
set { id = value; }
}
[XmlAttribute]
public string Name
{
get { return name; }
set { name = value; }
}

//[XmlAttribute] can't use on Complex Type
public Teachers1.Stude nts Students
{
get { return this.students; }
set { this.students = value; }
}

#endregion -------------------------------------------

#region Constructors +++++++++++++++ +++++++++++++++ +++

// empty constructor
public Class()
{ }

// Full Constructor
public Class(int Id, string Name, Teachers1.Stude nts
Students)
{
this.id = Id;
this.name = Name;
this.students = Students;
}

#endregion -------------------------------------------
}

#endregion

[Serializable()]
public class Classes : CollectionBase
{
#region Indexer +++++++++++++++ +++++++++++++++ +++

public Class this[int index]
{
set
{
List[index] = value;
}
get
{
return (Class)List[index];
}
}

#endregion ------------------------------------

#region Public Methods +++++++++++++++ +++++++++++

public int Add(Class value)
{
return List.Add(value) ;
}
public int IndexOf(Class value)
{
return List.IndexOf(va lue);
}
public void Insert(int index, Class value)
{
List.Insert(ind ex, value);
}
public void Remove(Class value)
{
List.Remove(val ue);
}
public bool Contains(Class value)
{
return List.Contains(v alue);
}

#endregion -------------------------------------------

#region Constructors +++++++++++++++ +++++++++++++++ +++

public Classes()
{ }

#endregion -------------------------------------------
}

#region Teacher
Object+++++++++ +++++++++++++++ +++++++++++++++ +++++++++++++++ ++

[Serializable()]
public class Teacher
{
#region Private members (Fields) +++++++++++++++ +++++

private int id;
private string name;
Teachers1.Class es classes = new Teachers1.Class es();

#endregion -------------------------------------------

#region Public Accessors (Properties) ++++++++++++++
//[XmlAttribute(At tributeName = "id", Namespace =
"urn:schema s-microsoft-com:xml-updategram")]
[XmlAttribute]
public int Id
{
get { return id; }
set { id = value; }
}
[XmlAttribute]
public string Name
{
get { return name; }
set { name = value; }
}

//[XmlAttribute] can't use on Complex Type
public Teachers1.Class es Classes
{
get { return this.classes; }
set { this.classes = value; }
}

#endregion -------------------------------------------

#region Constructors +++++++++++++++ +++++++++++++++ +++

// Default Constructor
public Teacher()
{ }

// Constructor 2
public Teacher(int id, string Name, Teachers1.Class es
Classes)
{
this.id = Id;
this.name = Name;
this.classes = Classes;
}

#endregion -------------------------------------------
}

#endregion

[Serializable()]
public class Teachers : CollectionBase
{
#region Private members (Fields) +++++++++++++++

#endregion --------------------------------------

#region Indexer +++++++++++++++ +++++++++++++++ +++

public Teacher this[int index]
{
set
{
List[index] = value;
}
get
{
return (Teacher)List[index];
}
}

#endregion ------------------------------------

#region Public Methods +++++++++++++++ +++++++++++

public int Add(Teacher value)
{
return List.Add(value) ;
}
public int IndexOf(Teacher value)
{
return List.IndexOf(va lue);
}
public void Insert(int index, Teacher value)
{
List.Insert(ind ex, value);
}
public void Remove(Teacher value)
{
List.Remove(val ue);
}
public bool Contains(Teache r value)
{
return List.Contains(v alue);
}
#endregion --------------------------------------

#region Constructors +++++++++++++++ +++++++++++++

public Teachers() { }

#endregion ---------------------------------------
}
}

=============== =============== =============== =

"Jon Skeet [C# MVP]" wrote:
[color=blue]
> a <a@discussions. microsoft.com> wrote:[color=green]
> > I'm having trouble testing a custom object. I've tried many different
> > approaches. One is shown below.
> >
> > The XML below shows the state of the object and I'm trying to test for that
> > state, ie there are NO classes in the Classes collection (Classes) of the
> > custom object.
> >
> > The conditional expression returns the error shown below it...[/color]
>
> <snip>
>
> How about
>
> if (Profile.Teache rs[0].Classes.Count= =0)
>
> It's hard to say exactly whether that'll work or not without knowing
> what the type of the property "Classes" is, but it's worth a try...
>
> --
> Jon Skeet - <skeet@pobox.co m>
> http://www.pobox.com/~skeet Blog: http://www.msmvps.com/jon.skeet
> If replying to the group, please do not mail me too
>[/color]

**Jon Skeet [C# MVP]** · Feb 24 '06, 09:25 PM

Re: Conditional Expression Testing for Nulls

a <a@discussions. microsoft.com> wrote:[color=blue]
> I tried that, as well as everything else, but no go.
>
> The Count method does not exist on the Teahcers object, just on the Teacer
> object.[/color]

I wasn't calling it on either of those though - I was calling it on a
Classes object, which does have a Count method.

If you want to know whether there are any *teachers*, use

if (Profile.Teache rs.Count==0)

Otherwise, the code I gave should show whether the first teacher has 0
classes.

--
Jon Skeet - <skeet@pobox.co m>
http://www.pobox.com/~skeet Blog: http://www.msmvps.com/jon.skeet
If replying to the group, please do not mail me too

a · Feb 24 '06, 10:55 PM

Re: Conditional Expression Testing for Nulls

Jon:

OK, I tested what you suggested and I evidently have another problem (it may
be with the indexer).

I tried this code to build all the objects at the same time and I get the
same error as previously posted. Only the first object is built.

Profile.Teacher s.Add(new Teachers1.Teach er());
// Add New Teacher to Profile
Profile.Teacher s[0].Classes.Add(ne w Teachers1.Class ());
// Add New Class
Profile.Teacher s[0].Classes[0].Students.Add(n ew
Teachers1.Stude nt()); // Add New Student

----------------------------------------------------------------

<?xml version="1.0" encoding="utf-16"?>
<ArrayOfTeacher s xmlns:xsi="http ://www.w3.org/2001/XMLSchema-instance"
xmlns:xsd="http ://www.w3.org/2001/XMLSchema">
<Teacher Id="0" Name="Bob">
<Classes />
</Teacher>
</ArrayOfTeachers >

I'm going to try to restructure my code to see if I can avoid this, if I
can't, I'll ask another question later, but thank you for verifying that
other code for me.

Paul
=============== =============== ===============

"Jon Skeet [C# MVP]" wrote:
[color=blue]
> a <a@discussions. microsoft.com> wrote:[color=green]
> > I tried that, as well as everything else, but no go.
> >
> > The Count method does not exist on the Teahcers object, just on the Teacer
> > object.[/color]
>
> I wasn't calling it on either of those though - I was calling it on a
> Classes object, which does have a Count method.
>
> If you want to know whether there are any *teachers*, use
>
> if (Profile.Teache rs.Count==0)
>
> Otherwise, the code I gave should show whether the first teacher has 0
> classes.
>
> --
> Jon Skeet - <skeet@pobox.co m>
> http://www.pobox.com/~skeet Blog: http://www.msmvps.com/jon.skeet
> If replying to the group, please do not mail me too
>[/color]

Conditional Expression Testing for Nulls

Conditional Expression Testing for Nulls

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