Take the blame, it's yours to take.
The most important thing in solving such problems is in realizing it is your problem. You can do little or anything about things by making the outcome dependent on others. Another good side to this policy is the fact that it makes you able to accept the hard to enjoy choices of others without holding a grudge. The blame is the most beautiful gift to yourself.
It's perfectly all right to do so and it sets your mind in the right direction. Even the options you generate outside the company may result in solutions inside the company. Even if the options you're trying to create are really far out.
Consider the general nature of what you communicate.
I personally don't think repetitive mentioning of the negative aspects of your situation to others, will do a whole lot other than nag. Compare the response you would get when telling your team leader about how unsatisfied you are with the lack of tasks you like, with his response if you told him how excited you are over an offer you got. Keep in mind; The companies decision to not honor your right to C# tasks, has been taken a long time ago. Set your mind to change and it will set theirs.
Make sure you are happy with what you are doing.
Whether you enjoy your work or not, will always depend on the context in which you do it. The work itself brings no joy, C# or no C#. Cleaning toilet bowls is a whole lot less unattractive if you know you are making ten grand an hour or saving the world. Whichever makes your clock tick. If you are not enjoying your work, its not because the work is bad, but because it doesn't bring you enough life quality. If that is not in the job, it may be found in the reasons you do it.
A job is no marriage.
When you work for a company, your goal is to improve that companies success. In return the company offers you whatever you agreed on,... or not, like in your case. You should feel no emotional obligation to that company or your colleagues, because that is not what a job is all about and the way the company observes its obligations to you shows that. They're not even living up to what you agreed on. As much as you may feel obliged to colleagues, you are in fact only obliged to live up to your agreement. This is why the pursuit of happiness is a human right. You don't owe it to anyone to give that up.
You are a business.
At the end of the day, you're on your own. People who do paid labor tend to forget that. Observe yourself as if you are a company of its own. That is the first one that needs success. Keep track of the state of its inventory, its growth rate, the qualities of its products, its survival chances, and so on and so forth. If C# is your product, C# is what your company should sell.
Everything you will ever see that is not untouched nature, was once no more than an idea in someone's head. That should begin to give you a notion of how powerful (your) thoughts really are. They are the only truly creative actions, the rest is reaction. They set things in motion. In controlling your thoughts, try not to focus on the downsides of your present situation. That is what you want to get away from, so technically, it should be behind you soon. There is no point in thinking about it. Instead try imagine what you would want to achieve and mentally elaborate on that. As a general rule, if it feels good to think about it, it is good.
You are the one that got you in to this situation and you are the one who will get you out of it. In this process, it is incredibly helpful for you to know who you are. Write about what bothers you and then read what you have written. Read what you wrote a month or a year ago. Watch yourself change. Make sound recordings and listen to yourself. Make videos and look at yourself. Get to know you as others know you. There is no faster way to detect and solve weaknesses in your own conduct than by using yourself as a source for feedback to find out why you do the things you do. Remember that behind every weakness lures a talent. Fight the weakness and you will suppress the talent. Overcome the weakness by embracing it and the talent will bloom.