Fuzz.cs

[Fact(Skip = "This test generator is capable of producing crashes that are not yet fixed; see, for example, Fuzz5815")]
public void Fuzz()
{
    int dt = Math.Abs(new DateTime().Ticks.GetHashCode() % 10000000);

    // generate a pattern-matching switch randomly from templates
    string[] expressions = new[]
    {
        "M",              // a method group
        "(() => 1)",      // a lambda expression
        "1",              // a constant
        "2",              // a constant
        "null",           // the null constant
        "default(int?)",  // a null constant of type int?
        "((int?)1)",      // a constant of type int?
        "M()",            // a method invocation
        "double.NaN",     // a scary constant
        "1.1",            // a double constant
        "NotFound"        // an unbindable expression
    };
    string[] types = new[]
    {
        "object",
        "var",
        "int",
        "int?",
        "double",
        "string",
        "NotFound"
    };

    // include casted method calls of the provided types
    expressions = types.Select(t => "((" + t + ")M())").Concat(expressions).ToArray();

    // forms:
    //   {expression}
    //   {type} x{0-9}
    //   {type} _
    string[] patterns = expressions
        .Concat(types.SelectMany(t => Enumerable.Range(0, 10).Select(i => t + " x" + i)))
        .Concat(types.Select(t => t + " _"))
        .ToArray();

    for (int i = 1; i < 100000; i++)
    {
        PatternMatchingFuzz(i + dt, expressions, patterns);
    }
}

public void PatternMatchingFuzz(int dt, string[] expressions, string[] patterns)
{
    Random r = new Random(dt);

    string Expression() => expressions[r.Next(expressions.Length)];
    string Pattern() => patterns[r.Next(patterns.Length)];
           
    const string body = @"
public class Program{0}
{{
public static void Main(string[] args)
{{
{1}
}}
private static object M() => null;
}}";
    var statement = new StringBuilder();
    switch (r.Next(2))
    {
        case 0:
            // test the "is-pattern" expression
            statement.AppendFormat("if ({0} is {1}) {{}}", Expression(), Pattern());
            break;
        case 1:
            // test the pattern switch statement
            statement.AppendFormat("switch ({0})\n{{\n", Expression());
            var nCases = r.Next(5);
            for (int i = 1; i <= nCases; i++)
            {
                statement.AppendFormat("    case {0}:\n", Pattern());
                if (i == nCases || r.Next(2) == 0)
                {
                    statement.AppendLine("        break;");
                }
            }
            statement.AppendLine("}");
            break;
        default:
            throw null;
    }
    var program = string.Format(body, dt, statement);
    CreateCompilationWithMscorlib45(program).GetDiagnostics();
}