README.md

NullRefChecksAnalyzer

The extension provides analysis and code fix to find and remove checks for arguments of reference types on null values.

Analyzer

The analyzer covers the arguments of syntax nodes such as method, constructor, and indexer.

The analyzer consists of subclasses-analyzers that run analysis for every possible type of expression that contains a null check of reference type parameters.

Examples of covered checks:

class Program
    {
        public bool this[string ss]
        {
            get
            {
                return ss == null; // warning
            }
        }

        public Program(string a, string b)
        {
            if (b == null || !(a == null) && b == "a") { } // warning on the 1st and 2nd expression
            if (a != null || a == "a") { } // warning on the 1st expression
            string[] args = new string[] { };
            if (args == null) { } // no warning
        }
        
        static void Main(string[] args, int c)
        {
            if (c == null) { } // no warning because of non-reference parameter type
            if (args == null) { } // warning
            if (!(args == null)) { } // warning
            else if (c == 1) { } // no warning
            if (!(null == args)) { } // warning
            if (!(args == default)) { } // warning
            if (args is null) { } // warning
            if (args is { }) { } // warning
            if (args is null or { }) { } // warning
            if (args is not null or null || c == 1) { } // warning on the 1st expression
            if (args.Equals(null)) { } // warning
            if (args.Equals(default)) { } // warning
            if (ReferenceEquals(args, null)) { } // warning
            if (ReferenceEquals(null, args)) { } // warning

            var z = args == null; // warning
            var w = args != null; // warning

            Func<string[], bool> f1 = (args) => args == null; // warning
            Func<string[], bool> f2 = (args) => args is null; // warning
            Func<string[], bool> f3 = (args) => args is { }; // warning
            Func<string[], bool> f4 = (args) => args is not null; // warning

            args?.ToString(); // warning
            var x = args == null ? 1 : 2; // warning
            var y = args is null ? 3 : 4; // warning

            var a = args ?? new object(); // warning
            var b = args ??= new string[] { "a", "b", "c" }; // warning

            switch (args)
            {
                case null: // warning
                    break;
                default:
                    break;
            }
        }
    }

Code fix

The code fixer works according to the algorithm for finding the "root" of the problem, which is signaled by the analyzer. It could be if-statement (in most cases), equals value clause expression, switch statement, etc. The "root" can also be just the body of the method/constructor.

Examples of covered changes provided by code fix:

Simple if-statements:

static void Main(string[] args, int c)
{
    if (c == null) { }
-   if (args == null) { }
}

Simple if-statements:

static void Main(string[] args, int c)
{
-   if (args != null) { }
+   if (true) { }
}

Composite if-statement:

static void Main(string[] args, int c)
{
-   if (args == null || c == 1)
+   if (c == 1) { }
}

static void Main(string[] args, int c)
{
    // 1st fix by 1st reported expression
-   if (!(b != null) || !(b == "a" && b == null)) { }
+   if (!(b == "a" && b == null)) { }
    // 2nd fix
-   if (!(b == "a" && b == null)) { }
+   if (!(false)) { }
}

One more example with logical expressions:

static void Main(string[] args, int c)
{
    // 1st fix
-   if (a != null && a == "a") { }
+   if (a == "a") { }
}

'Logical not' and the simpliest pattern matching expressions:

-   if (!!!(args == null)) { }
+   if (true) { }
-   if (!(null == args)) { }
+   if (true) { }
-   if (!(args == default)) { }
+   if (true) { }
-   if (args is null) { }
+   if (false) { }
-   if (args is not null) { }
+   if (true) { }

Switch statement:

switch (args)
{
-    case null:
-        break;
     default:
         break;
}

Conditional access expression:

• without any specified parent statement

-   args?.ToString();
+   args.ToString();

• in if-statement

-   if (args?.ToString() == "fff") { }
+   if (args.ToString() == "fff") { }

• with equals value clause expressions

-   var abc = args?.ToString();
+   var abc = args.ToString();

Equals value clause expressions:

-   var z = args == null;
-   var w = args != null;
+   var z = false;
+   var w = true;

Invocation expressions:

• in if-statement

-   if (!object.ReferenceEquals(args, null)) { }
+   if (true) { }
-   if (ReferenceEquals(null, args)) { }
+   if (false) { }

• without any specified parent statement

-   ReferenceEquals(args, null);

• with equals value clause expressions

-   var x = (!(object.ReferenceEquals(args, null))) ? 1 : 2;
+   if (true) { }
-   var y = ReferenceEquals(args, null) ? 3 : 4;
+   if (false) { }

Coalesce expressions:

-   var a = args ?? new object() ?? new object();
+   var a = args;
-   var b = args ??= (args ??= new string[] { "a", "b", "c" });
+   var b = args;

In return statements:

-   return args ?? null;
+   return args;

-   return args ??= null;
+   return args;

-   return args != null;
+   return true;

-   return args == null;
+   return false;

-   return (!(args == null));
+   return true;

Not implemented

TL:DR: pattern matching expressions are mostly not implemented

Examples of not implemented code fixes:

if (args is { } || true) { }
if (args is null or not { }) { }
if (args is not null or null || c == 1) { } 

Func<string[], bool> f2 = (args) => args is null;
Func<string[], bool> f3 = (args) => args is { };
Func<string[], bool> f4 = (args) => args is not null;

if (args == null)
{
    // some body (once told me...)
}
else
{
    // should be saved, but actually deleted along with the entire if-statement expression
    // same situation with if/else if/else statements
}

var a = args switch
{
    null => 1,
    _ => 2
};