Benchmark object String Literal string.Intern StringBuilder

`Object.ReferenceEquals`

The C# object.ReferenceEquals method compares references. When you allocate an object, you receive a reference containing a value indicating its memory location.

More details

The object.ReferenceEquals method gives you the ability to determine if 2 objects are in the same memory location. The data pointed to is not compared.

Example

Two StringBuilders are allocated upon the managed heap. The identifiers builder1 and builder2 are reference variables. They contain values that points to those locations on the heap.

Detail The first call to object.ReferenceEquals returns false because those 2 memory locations are different.

Detail The second call returns true because the references were set equal in an assignment.

Info Assignments in the C# language perform a simple bitwise copy of the value. This makes the reference equivalent.

Detail The two references literal1 and literal2 are equal. They point to the same memory location. String literals are pooled.

using System;
using System.Text;

class Program
{
    static void Main()
    {
        // Test object.ReferenceEquals.
        StringBuilder builder1 = new StringBuilder();
        StringBuilder builder2 = new StringBuilder();
        Console.WriteLine(object.ReferenceEquals(builder1, builder2));
        builder1 = builder2;
        Console.WriteLine(object.ReferenceEquals(builder1, builder2));

        // Test object.ReferenceEquals on string literals.
        string literal1 = "a";
        string literal2 = "a";
        Console.WriteLine(object.ReferenceEquals(literal1, literal2));
        literal1 = literal2;
        Console.WriteLine(object.ReferenceEquals(literal1, literal2));
    }
}False
True
True
True

Performance

Is object.ReferenceEquals a slow or fast method call? I developed this console program that tests the performance of object.ReferenceEquals on a custom class.

Info ReferenceEquals was fast. It was a faster way of testing two objects for equality than comparing a unique Id field.

Thus With objects similar to A, object.ReferenceEquals is not slow. We can use it without too much concern.

using System;
using System.Diagnostics;

class A
{
    public int Id;
}

class Program
{
    const int _max = 1000000000;
    static void Main()
    {
        bool same = true;
        A a1 = new A() { Id = 1 };
        A a2 = same ?
            a1 :
            new A() { Id = 2 };

        int x = 0;
        var s1 = Stopwatch.StartNew();
        for (int i = 0; i < _max; i++)
        {
            if (object.ReferenceEquals(a1, a2))
            {
                x++;
            }
        }
        s1.Stop();
        var s2 = Stopwatch.StartNew();
        for (int i = 0; i < _max; i++)
        {
            if (a1.Id == a2.Id)
            {
                x++;
            }
        }
        s2.Stop();
        Console.WriteLine(((double)(s1.Elapsed.TotalMilliseconds * 1000000) / _max).ToString("0.00 ns"));
        Console.WriteLine(((double)(s2.Elapsed.TotalMilliseconds * 1000000) / _max).ToString("0.00 ns"));
    }
}0.32 ns [object.ReferenceEquals]
0.64 ns0.64 ns [object.ReferenceEquals]
0.64 ns

References in the C# language refer to a location in memory. By comparing references with ReferenceEquals, we determine if 2 variables have an identical location in memory.