Question

i have some mandatory fields and when calling the constructor of my class i got null reference exception, how prevent this please.

new Part(Partitems["Name"].ToString(), Partitems["Logo"].ToString(), Partitems["Description"].ToString(), Partitems["URL"].ToString()));

MY Class :

public class Part
    {
        string _Name= string.Empty;

        public string Name
        {
            get { return _Name; }
            set { _Name = value; }
        }

…
..

EDIT
Here is constructor code

public Part(string Name, string Logo, string Description, string URL) 
{ 
    this.Name = Name; 
    this.Logo = Logo; 
    this.Description = Description; 
    this.URL = URL; 
} 

Answer 1

There’s nothing in the constructor that could cause a null reference exception, as assigning null to a string property is perfectly valid. So, either your Partitems variable is null, or one of it’s properties returns null.

Example for handling both situations:

if (Partitems != null) {
  object name = Partitems["Name"] ?? String.Empty;
  object logo = Partitems["Logo"] ?? String.Empty;
  object description = Partitems["Description"] ?? String.Empty;
  object url = Partitems["URL"] ?? String.Empty;
  part = new Part(name.ToString(), logo.ToString(), description.ToString(), url.ToString()));
} else {
  // Oops, Partitems is null. Better tell the user or log the error...
}

This will replace any null values in the properties with empty strings. However, you might want to handle it as an error situation instead, depending on the situation.