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.
When calling StringBuilder.Append with a unit length string, consider using a const char rather than a unit length const string to improve performance.
This rule flags the Count and LongCount LINQ method calls used to check if the collection has at least one element. These method calls require enumerating the entire collection to compute the count. The same check is faster with the Any method as it avoids enumerating the collection.
Comparing strings using the String.Length property or the String.IsNullOrEmpty method is faster than using Equals. This is because Equals executes significantly more MSIL instructions than either IsNullOrEmpty or the number of instructions executed to retrieve the Length property value and compare it to zero.
The .NET runtime initializes all fields of reference types to their default values before running the constructor. In most cases, explicitly initializing a field to its default value in a constructor is redundant, adding maintenance costs and potentially degrading performance
The value of a const field is computed at compile time and stored in the metadata, which improves run-time performance when it is compared to a static readonly field.
Mark McDowall