As Selman22 said, you are overriding the default object.Equals method, which accepts an object obj and not a safe compile time type.
In order for that to happen, make your type implement IEquatable<Box>:
public class Box : IEquatable<Box>
{
double height, length, breadth;
public static bool operator ==(Box obj1, Box obj2)
{
if (ReferenceEquals(obj1, obj2))
{
return true;
}
if (ReferenceEquals(obj1, null))
{
return false;
}
if (ReferenceEquals(obj2, null))
{
return false;
}
return obj1.Equals(obj2);
}
public static bool operator !=(Box obj1, Box obj2)
{
return !(obj1 == obj2);
}
public bool Equals(Box other)
{
if (ReferenceEquals(other, null))
{
return false;
}
if (ReferenceEquals(this, other))
{
return true;
}
return height.Equals(other.height)
&& length.Equals(other.length)
&& breadth.Equals(other.breadth);
}
public override bool Equals(object obj)
{
return Equals(obj as Box);
}
public override int GetHashCode()
{
unchecked
{
int hashCode = height.GetHashCode();
hashCode = (hashCode * 397) ^ length.GetHashCode();
hashCode = (hashCode * 397) ^ breadth.GetHashCode();
return hashCode;
}
}
}
Another thing to note is that you are making a floating point comparison using the equality operator and you might experience a loss of precision.