Pass Class As Parameter Having Parameterized Constructor

Depends on how you "pass Company class to CompanyDetails method".

Because you have declared a constructor with a parameter, the system no longer generates a parameterless constructor for you automatically.
So if you try to create a new instance of your Company class without passing it a string it looks for a parameterless version and reports an error.

There are a couple of ways round this:
1) If the name is compulsory, then leave it as it is, and provide a new when you create the new instance.
2) Convert your string to an optional parameter:

public CompanyName(string name = "Anonymous")
{
  this._name = name;
}

3) Write a parameterless constructor that sets a default string value.

I tried your logic its just work fine...

namespace ConsoleApplication3
{


    public class Company
    {
    private string _name;
   
//BEFORE
// public CompanyName(string name)
// {
//  this._name = name;
// }
    public Company(string name)
    {
    this._name = name;
    }
} 


    class Program
    {
        public string CompanyDetails(Company objcompany)
          {
            //Some Logic
         return "hello";
         }
        static void Main(string[] args)
        {
            Company ob = new Company("sasank");
            Program obj = new Program();
            //Passing Company objects to CompanyDetails method
            Console.WriteLine(obj.CompanyDetails(ob));
          // o/p  hello
        }
    }
}