Avoid implicitly allocating params arrays in loops

Find calls to System.* methods inside loops, where those methods take params arrays and those params arrays are being allocated, and hoist the allocation to before the loop if possible.

Category: Performance

Estimates:

Analyzer: Medium
Fixer: Large

@terrajobst

@terrajobst can you please provide a code example?

Suggested severity: Info
Suggested category: Performance
Suggested milestone: Future

Examples

Let's use the Console.WriteLine overload that takes a params array:

public static class Console
{
  public static void WriteLine(string format, params object?[]? arg);
}

Case 1

Flag

Before:

int i = 0;
while (i++ < 10)
{
    Console.WriteLine("{0}, {1}, {2}, {3}", 1, 2, 3, 4);
}

After:

int i = 0;
object[] arg = new object[] { 1, 2, 3, 4 };
while (i++ < 10)
{
    Console.WriteLine("{0}, {1}, {2}, {3}", arg);
}

Case 2

Do not flag: An element in the passed params array has variable data

int i = 0;
while (i++ < 10)
{
    Console.WriteLine("{0}, {1}, {2}, {3}", i, 2, 3, 4);
}

Case 3

Flag

Before: Variables are used inside the loop, but they weren't modified between their creation and their use:

short a = 10, b = 20, c = 30, d = 40;
int i = 0;
while (i++ < 10)
{
    Console.WriteLine("{0}, {1}, {2}, {3}", a, b, c, d);
}

After:

short a = 10, b = 20, c = 30, d = 40;
int i = 0;
object[] arg = new object[] { a, b, c, d };
while (i++ < 10)
{
    Console.WriteLine("{0}, {1}, {2}, {3}", arg);
}

Case 4

Having this class:

class C
{
    private string _name;
    public C(string name) => _name = name;
    public override string ToString() => _name;
}

Flag

Before:

int i = 0;
while (i++ < 10)
{
    Console.WriteLine("Format {0}, {1}", new C("a"), new C("b"), new C("c"), new C("d"));
}

After: Notice the data type of the array can be preserved as object[]

int i = 0;
object[] arg = new object[] { new C("a"), new C("b"), new C("c"), new C("d") };
while (i++ < 10)
{
    Console.WriteLine("{0}, {1}, {2}, {3}", arg);
}

Case 5

Do not flag: An element in the passed params array has variable data

int i = 0;
while (i++ < 10)
{
    Console.WriteLine("{0}, {1}, {2}, {3}", new C($"{i}"), new C("b"), new C("c"), new C("d"));
}

Case 6

Do not flag: Method does not belong to `System.*`

class Program
{
    static void Main()
    {
        int i = 0;
        while (i++ < 10)
        {
            MyOwnMethod(1, 2, 3, 4);
        }
    }

    static void MyOwnMethod(params object[] myargs){ }
}

Things to consider:

We need to check that the array values do not get modified between iterations.
If the analyzer flags a case, the fixer should create the array immediately before the beginning of the loop, using the data type of the method's params array parameter.
The new array should be named after the method parameter's name, unless a variable with that name exist, then append a number.
Restrict the analyzer to only check invocations of methods that live inside System.*.
CS0231: params is always the last parameter in a method signature, so the analyzer should only check if the last parameter is a params array.

I would like to be assigned to this, please.

I think this is ALREADY implemented (except the "in loop" part and the attribute requirement).

https://github.com/dotnet/roslyn-analyzers/blob/a46689a39bd6bbea9761961b4683bb85c84e3be8/src/PerformanceSensitiveAnalyzers/Microsoft.CodeAnalysis.PerformanceSensitiveAnalyzers.sarif#L12-L24

An implementation of this as a "CAxxxx" can benefit from the existing implementation.

With params ReadOnlySpan<T> coming, is this still valuable?

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Avoid implicitly allocating params arrays in loops #33793

Examples

Case 1

Flag

Case 2

Do not flag: An element in the passed params array has variable data

Case 3

Flag

Case 4

Flag

Case 5

Do not flag: An element in the passed params array has variable data

Case 6

Do not flag: Method does not belong to `System.*`

2 remaining items

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Avoid implicitly allocating params arrays in loops #33793

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jeffhandley commented on Mar 21, 2020

carlossanlop commented on Jan 8, 2021

carlossanlop commented on Mar 30, 2021

Examples

Case 1

Flag

Case 2

Do not flag: An element in the passed params array has variable data

Case 3

Flag

Case 4

Flag

Case 5

Do not flag: An element in the passed params array has variable data

Case 6

Do not flag: Method does not belong to System.*

2 remaining items

NewellClark commented on Sep 19, 2021

Youssef1313 commented on Jan 7, 2022

huoyaoyuan commented on Apr 27, 2024

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Do not flag: Method does not belong to `System.*`