Type confusion: A type is class, a variable and an object?

Question

I'm currently working through the Pluralsight C# 5.0 course, and I'm relatively new to programming.

I previously thought I understood the concept of Data Types on a basic level, Int/Array/Strings etc. In this course it starts to introduce C#'s huge emphasis on Types, and creating your own custom types.

One of the course code snippets which I've included below, is refusing to sink in and I was hoping someone could provide some clarity or a different way of thinking about it.

Program.cs:

GradeStatistics stats = book.ComputeStatistics();

GradeStatistics.cs:

namespace Grades
    {
        public class GradeStatistics
        {
            public GradeStatistics()
            {
                HighestGrade = 0;
                LowestGrade = float.MaxValue;
            }

            public float AverageGrade;
            public float HighestGrade;
            public float LowestGrade;
        }
    }

Grades:

public GradeStatistics ComputeStatistics()
        {
            GradeStatistics stats = new GradeStatistics();

            float sum = 0f;
            foreach (float grade in grades)
            {
                stats.HighestGrade = Math.Max(grade, stats.HighestGrade);
                stats.LowestGrade = Math.Min(grade, stats.LowestGrade);
                sum += grade;
            }

            stats.AverageGrade = sum / grades.Count;
            return stats;
        }

I'm finding it particularly difficult to understand what exactly GradeStatistics is. In the course it is referred to as not only a class, but as a variable, and furthermore also being returned as an object in Grades.

Any clarity is appreciated, as I'm finding it a little difficult to follow with all of the above terms being thrown around.

Answer 1

GradeStatistics is a class, from this declaration:

public class GradeStatistics

stats is a variable of type GradeStatistics, from this declaration:

GradeStatistics stats = new GradeStatistics();

The return type of ComputeStatistics is GradeStatistics, from this declaration:

public GradeStatistics ComputeStatistics()

So, it's a class. It is being used to declare a variable with a particular type, and it is used to declare what a particular method will return, an object of the type.

If it helps, you can sort of think of a type as a blueprint. You can have a blueprint of a house. This will tell you that "if you had a house", this is what it would look like.

When you construct an instance of the type, ie. build the house, you get an instance. It has a type (it follows the blueprint), but it may be different from other instances, having other property values (like the color of the paint used, or the style of doors).

Answer 2

First, GradeStatistics is a Class.

Second, it is not referred as a variable anywhere. But stats is actually a variable of type GradeStatistics in this line.

GradeStatistics stats = new GradeStatistics();

Third, ComputeStatistics is a function which return GradeStatistics.

And if you have read OOP than you should know that any object of any class type can be returned as a function return value.