Null reference exception error for the array of custom datetime class

Question

0.00/5 (No votes)

See more:

I have created a simple datetime class of my own like this.

public class datetime
 {


     private readonly long value = -1;

     public datetime(long value)
     {
         this.value = value;
     }

     public static implicit operator datetime(long value)
     {
         return new datetime(value);
     }




     public static bool operator ==(datetime datetimeLeftSide, datetime datetimeRightSide)
     {

         if (datetimeLeftSide.GetValue() == datetimeRightSide.GetValue())
         {
             return (true);
         }
         else
         {
             return (false);
         }
     }



     public static bool operator !=(datetime datetimeLeftSide, datetime datetimeRightSide)
     {

         return !(datetimeLeftSide == datetimeRightSide);
     }


     public long GetValue()
     {
         return (this.value);
     }


     public static implicit operator long(datetime first)
     {
         return first.value;
     }

 }

When I declare an array of datetime class like this

mydatetime = new datetime[3];

When I try to compare with another datetime variable

datetime dt = 1000;

if (mydatetime[1] != dt)   //I get the null reference exception here
{
   //do something
}

I get the null reference excpetion error with the array of datetime because I did not initilized the mydatetime array.

However, is there any work around to avoid this null reference exception error when I forgot to initialize the array before comparing.

I am looking for some ways that the values are automatically filled when array of datetime is just declared with the size, just for convenience.

What I have tried:

I tried to add this empty constructor to the datetime class. However, I still get the null reference exception error when the array is not filled with any values.

public datetime()
  {
      this.value = -1;
  }

Posted 24-Nov-20 23:47pm

auto9817

Updated 25-Nov-20 0:09am

v3

Add a Solution

2 solutions

Solution 1

No, you can't - your code is explicitly declaring an array of three datetime class instances, but the array itself does not contain the actual instances: it contains three references to those instances instead.

What you could do is make your class a struct instead as that is a value type instead of a reference type ... but ... that also has implications: Using struct and class - what's that all about?[^]

Posted 25-Nov-20 0:00am

OriginalGriff

Add a Solution

Add your solution here

Treat my content as plain text, not as HTML

Preview 0

…

Existing Members

Sign in to your account

...or Join us

Download, Vote, Comment, Publish.

Your Email
Password
Forgot your password?

Your Email
This email is in use. Do you need your password?
Optional Password

I have read and agree to the Terms of Service and Privacy Policy
Please subscribe me to the CodeProject newsletters

When answering a question please:

Read the question carefully.
Understand that English isn't everyone's first language so be lenient of bad spelling and grammar.
If a question is poorly phrased then either ask for clarification, ignore it, or edit the question and fix the problem. Insults are not welcome.
Don't tell someone to read the manual. Chances are they have and don't get it. Provide an answer or move on to the next question.

Let's work to help developers, not make them feel stupid.

This content, along with any associated source code and files, is licensed under The Code Project Open License (CPOL)

Richard Deeming · Accepted Answer · 2020-11-25T00:09:00

The error is thrown by your equality operator. You are calling a method on both operands, without testing whether they are null first. And since your type is a reference type, one or both sides could be null.

(You'll have the same problem with your implicit datetime to long conversion operator - the datetime parameter could be null, and you'll get a NullReferenceException.)

You could fix your operator:

C#

public static bool operator ==(datetime datetimeLeftSide, datetime datetimeRightSide)
{
    if (datetimeLeftSide is null) return datetimeRightSide is null;
    if (datetimeRightSide is null) return false;
    return datetimeLeftSide.GetValue() == datetimeRightSide.GetValue();
}

But you should really be providing the full set of equality methods:

C#

public class datetime : IEquatable<datetime>
{
    ...
    
    public override int GetHashCode()
    {
        return value.GetHashCode();
    }
    
    public override bool Equals(object obj)
    {
        return Equals(obj as datetime);
    }
    
    public bool Equals(datetime other)
    {
        if (other is null) return false;
        return other.value == this.value;
    }
    
    public static bool operator ==(datetime left, datetime right)
    {
        return Equals(left, right);
    }
    
    public static bool operator !=(datetime left, datetime right)
    {
        return !Equals(left, right);
    }
    
    ...
}

NB: If you want the values to be filled automatically, you would need to create a value type instead:
Structure types - C# reference | Microsoft Docs[^]

However, you would not be able to have a default value of -1 for the field.

C#

public readonly struct datetime : IEquatable<datetime>
{
    private readonly long value;
    
    public datetime(long value)
    {
        this.value = value;
    }
    
    public long GetValue()
    {
        return value;
    }
    
    public override int GetHashCode()
    {
        return value.GetHashCode();
    }
    
    public override bool Equals(object obj)
    {
        return obj is datetime other && Equals(other);
    }
    
    public bool Equals(datetime other)
    {
        return other.value == this.value;
    }
    
    public static bool operator ==(datetime left, datetime right)
    {
        return Equals(left, right);
    }
    
    public static bool operator !=(datetime left, datetime right)
    {
        return !Equals(left, right);
    }
    
    public static implicit operator datetime(long value)
    {
        return new datetime(value);
    }
    
    public static implicit operator long(datetime first)
    {
        return first.value;
    }
}