System.Linq.Expressions.Expression.PostDecrementAssign C# (CSharp) Method

        public static UnaryExpression PostDecrementAssign(Expression expression)
        {
            return MakeOpAssignUnary(ExpressionType.PostDecrementAssign, expression, method: null);
        }

        // Pseudocode:
        //string input =>
        //{
        //    R result = 0;
        //    for (int i = input.Length - 1; i >= 0; i--)
        //    {
        //        result <<= 6;
        //        var m = _invMap[input[i]];
        //        if (m == 0xff)
        //            return default(ConversionResult<R>);
        //        result += m;
        //    }
        //    return new ConversionResult<R>(result);
        //}
        private LambdaExpression fromLambda(Type to)
        {
            var stringthis = typeof(string).GetTypeInfo().DeclaredProperties.First(p => p.GetIndexParameters().Length == 1 && p.GetIndexParameters()[0].ParameterType == typeof(int));
            var input      = Ex.Parameter(typeof(string), "input");
            var result     = Ex.Parameter(to, "result");
            var i          = Ex.Parameter(typeof(int), "i");
            var m          = Ex.Parameter(typeof(byte), "m");
            var loopstart  = Ex.Label("loopstart");
            var end        = Ex.Label(typeof(ConversionResult <>).MakeGenericType(to), "end");
            var loop       = Ex.Block(
                Ex.Label(loopstart),
                Ex.IfThen(Ex.MakeBinary(ExpressionType.LessThan, i, Ex.Constant(0)),
                          Ex.Goto(end, Result(to, result))),
                Ex.LeftShiftAssign(result, Ex.Constant(6)),
                Ex.Assign(m, Ex.ArrayIndex(Ex.Constant(_invMap), Ex.Convert(Ex.MakeIndex(input, stringthis, new[] { i }), typeof(int)))),
                Ex.IfThen(Ex.MakeBinary(ExpressionType.Equal, m, Ex.Constant((byte)0xff)),
                          Ex.Goto(end, NoResult(to))),
                Ex.AddAssign(result, Ex.Convert(m, result.Type)),
                Ex.PostDecrementAssign(i),
                Ex.Goto(loopstart));
            var block = Ex.Block(new[] { result, i, m },
                                 Ex.Assign(result, Ex.Convert(Ex.Constant(0), to)),
                                 Ex.Assign(i, Ex.MakeBinary(ExpressionType.Subtract, Ex.Property(input, nameof(string.Length)), Ex.Constant(1))),
                                 loop,
                                 Ex.Label(end, NoResult(to)));

            return(Ex.Lambda(block, input));
        }