CA1845: Use span-based 'string.Concat'

#Cause

This rule locates string-concatenation expressions that contain System.String.Substring calls and suggests replacing System.String.Substring with System.MemoryExtensions.AsSpan and using the span-based overload of System.String.Concat.

#Rule description

Calling Substring produces a copy of the extracted substring. By using AsSpan instead of Substring and calling the overload of string.Concat that accepts spans, you can eliminate the unnecessary string allocation.

#How to fix violations

To fix violations:

1. Replace the string concatenation with a call to string.Concat, and
2. Replace calls to Substring with calls to AsSpan.

The following code snippet shows examples of violations, and how to fix them.

using System;

class Example
{
    public void Method()
    {
        string text = "fwobz the fwutzle";

        //  Violation: allocations by Substring are wasteful.
        string s1 = text.Substring(10) + "---" + text.Substring(0, 5);

        //  Fixed: using AsSpan avoids allocations of temporary strings.
        string s2 = string.Concat(text.AsSpan(10), "---", text.AsSpan(0, 5));
    }
}

#When to suppress warnings

Do not suppress warnings from this rule. There is no reason to use Substring over AsSpan when the extracted substring is only being passed to a method with a span-based equivalent.

#See also

• Performance rules