“Object reference not set to an instance of an object.” error

Question

public override bool Equals(object obj)
{
    var par = obj as Parameter;
    if (ParameterName == par.ParameterName )  /* <=throws error at this code */
        return true;
    else
        return false;

}

I am getting this error when I am running that code: Object reference not set to an instance of an object

How to remove the error in c#?

Answer 1

Principle ONE of defensive programming - always check for NULL !

public override bool Equals(object obj)
{
    // this statement CAN FAIL if your "obj" is *NOT* of type "Parameter"
    var par = obj as Parameter;

    // so therefore you MUST be ready to deal with a NULL value here!
    if (par != null && ParameterName == par.ParameterName)  
    {
        return true;
    }
    else
    {
        return false;
    }

}

Answer 2

You can check if par is not null before accessing ParameterName

public override bool Equals(object obj)
{
    var par = obj as Parameter;
    if (par!= null && ParameterName == par.ParameterName )  /* <=throws error at this code */
        return true;
    else
        return false;    
}

Answer 3

How about checking if it is null first?

public override bool Equals(object obj)
{
    var par = obj as Parameter;
    if (par != null && ParameterName == par.ParameterName )  /* <=throws error at this code */
        return true;
    else
        return false;

}

Answer 4

ther'es a problem in your convertion to Parametre you have to check if null

public override bool Equals(object obj)
{
   var par = obj as Parameter;
   if (par  != null && ParameterName == par.ParameterName ) 
      return true;
   else
      return false;

}

Answer 5

use this for checking par != null null

public override bool Equals(object obj)
{
   var par = obj as Parameter;
   if (par  != null && ParameterName == par.ParameterName ) 
      return true;
   else
      return false;

}