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What's the difference between System.ValueTuple and System.Tuple?

Posted by: admin November 30, 2017 Leave a comment

Questions:

I decompiled some C# 7 libraries and saw ValueTuple generics being used. What are ValueTuples and why not Tuple instead?

Answers:

What are ValuedTuples and why not Tuple instead?

A ValueTuple is a struct which reflects a tuple, same as the original System.Tuple class.

The main difference between Tuple and ValueTuple are:

  • System.ValueTuple is a value type (struct), while System.Tuple is a reference type (class). This is meaningful when talking about allocations and GC pressure.
  • System.ValueTuple isn’t only a struct, it’s a mutable one, and one has to be careful when using them as such. Think what happens when a class holds a System.ValueTuple as a field.
  • System.ValueTuple exposes its items via fields instead of properties.

Until C# 7, using tuples wasn’t very convenient. Their field names are Item1, Item2, etc, and the language hadn’t supplied syntax sugar for them like most other languages do (Python, Scala).

When the .NET language design team decided to incorporate tuples and add syntax sugar to them at the language level an important factor was performance. With ValueTuple being a value type, you can avoid GC pressure when using them because (as an implementation detail) they’ll be allocated on the stack.

Additionally, a struct gets automatic (shallow) equality semantics by the runtime, where a class doesn’t. Although the design team made sure there will be an even more optimized equality for tuples, hence implemented a custom equality for it.

Here is a paragraph from the design notes of Tuples:

Struct or Class:

As mentioned, I propose to make tuple types structs rather than
classes, so that no allocation penalty is associated with them. They
should be as lightweight as possible.

Arguably, structs can end up being more costly, because assignment
copies a bigger value. So if they are assigned a lot more than they
are created, then structs would be a bad choice.

In their very motivation, though, tuples are ephemeral. You would use
them when the parts are more important than the whole. So the common
pattern would be to construct, return and immediately deconstruct
them. In this situation structs are clearly preferable.

Structs also have a number of other benefits, which will become
obvious in the following.

Examples:

You can easily see that working with System.Tuple becomes ambiguous very quickly. For example, say we have a method which calculates a sum and a count of a List<Int>:

public Tuple<int, int> DoStuff(IEnumerable<int> ints)
{
    var sum = 0;
    var count = 0;

    foreach (var value in values) { sum += value; count++; }

    return new Tuple(sum, count);
}

On the receiving end, we end up with:

Tuple<int, int> result = DoStuff(Enumerable.Range(0, 10));

// What is Item1 and what is Item2?
// Which one is the sum and which is the count?
Console.WriteLine(result.Item1);
Console.WriteLine(result.Item2);

The way you can deconstruct value tuples into named arguments is the real power of the feature:

public (int sum, int count) DoStuff(IEnumerable<int> values) 
{
    var res = (sum: 0, count: 0);
    foreach (var value in values) { res.sum += value; res.count++; }
    return res;
}

And on the receiving end:

var result = DoStuff(Enumerable.Range(0, 10));
Console.WriteLine($"Sum: {result.Sum}, Count: {result.Count}");

Or:

var (sum, count) = DoStuff(Enumerable.Range(0, 10));
Console.WriteLine($"Sum: {sum}, Count: {count}");

Compiler goodies:

If we look under the cover of our previous example, we can see exactly how the compiler is interpreting ValueTuple when we ask it to deconstruct:

[return: TupleElementNames(new string[] {
    "sum",
    "count"
})]
public ValueTuple<int, int> DoStuff(IEnumerable<int> values)
{
    ValueTuple<int, int> result;
    result..ctor(0, 0);
    foreach (int current in values)
    {
        result.Item1 += current;
        result.Item2++;
    }
    return result;
}

public void Foo()
{
    ValueTuple<int, int> expr_0E = this.DoStuff(Enumerable.Range(0, 10));
    int item = expr_0E.Item1;
    int arg_1A_0 = expr_0E.Item2;
}

Internally, the compiled code utilizes Item1 and Item2, but all of this is abstracted away from us since we work with a decomposed tuple. A tuple with named arguments gets annotated with the TupleElementNamesAttribute. If we use a single fresh variable instead of decomposing, we get:

public void Foo()
{
    ValueTuple<int, int> valueTuple = this.DoStuff(Enumerable.Range(0, 10));
    Console.WriteLine(string.Format("Sum: {0}, Count: {1})", valueTuple.Item1, valueTuple.Item2));
}

Note that the compiler still has to make some magic happen (via the attribute) when we debug our application, as it would be odd to see Item1, Item2.

Questions:
Answers:

The difference between Tuple and ValueTuple is that Tuple is a reference type and ValueTuple is a value type. The latter is desirable because changes to the language in C# 7 have tuples being used much more frequently, but allocating a new object on the heap for every tuple is a performance concern, particularly when it’s unnecessary.

However, in C# 7, the idea is that you never have to explicitly use either type because of the syntax sugar being added for tuple use. For example, in C# 6, if you wanted to use a tuple to return a value, you would have to do the following:

public Tuple<string, int> GetValues()
{
    // ...
    return new Tuple(stringVal, intVal);
}

var value = GetValues();
string s = value.Item1; 

However, in C# 7, you can use this:

public (string, int) GetValues()
{
    // ...
    return (stringVal, intVal);
}

var value = GetValues();
string s = value.Item1; 

You can even go a step further and give the values names:

public (string S, int I) GetValues()
{
    // ...
    return (stringVal, intVal);
}

var value = GetValues();
string s = value.S; 

… Or deconstruct the tuple entirely:

public (string S, int I) GetValues()
{
    // ...
    return (stringVal, intVal);
}

var (S, I) = GetValues();
string s = S;

Tuples weren’t often used in C# pre-7 because they were cumbersome and verbose, and only really used in cases where building a data class/struct for just a single instance of work would be more trouble than it was worth. But in C# 7, tuples have language-level support now, so using them is much cleaner and more useful.

Questions:
Answers:

I looked at the source for both Tuple and ValueTuple. The difference is that Tuple is a class and ValueTuple is a struct that implements IEquatable.

That means that Tuple == Tuple will return false if they are not the same instance, but ValueTuple == ValueTuple will return true if they are of the same type and Equals returns true for each of the values they contain.

Questions:
Answers:

Other answers forgot to mention important points.Instead of rephrasing, I’m gonna reference the XML documentation from source code:

The ValueTuple types (from arity 0 to 8) comprise the runtime implementation that underlies
tuples in C# and struct tuples in F#.

Aside from created via language syntax, they are most easily created via the
ValueTuple.Create factory methods.
The System.ValueTuple types differ from the System.Tuple types in that:

  • they are structs rather than classes,
  • they are mutable rather than readonly, and
  • their members (such as Item1, Item2, etc) are fields rather than properties.

With introduction of this type and C# 7.0 compiler, you can easily write

(int, string) idAndName = (1, "John");

And return two values from a method:

private (int, string) GetIdAndName()
{
   //.....
   return (id, name);
}

Contrary to System.Tuple you can update its members (Mutable) because they are public read-write Fields that can be given meaningful names:

(int id, string name) idAndName = (1, "John");
idAndName.name = "New Name";