Why does C# disallow readonly local variables?

I think it's a poor judgement on part of C# architects. readonly modifier on local variables helps maintain program correctness (just like asserts) and can potentially help the compiler optimize code (at least in the case of other languages). The fact that it's disallowed in C# right now, is another argument that some of the "features" of C# are merely an enforcement of personal coding style of its creators.

A proposal readonly locals and parameters for was briefly discussed by the C# 7 design team. From C# Design Meeting Notes for Jan 21, 2015:

Parameters and locals can be captured by lambdas and thereby accessed concurrently, but there's no way to protect them from shared-mutual-state issues: they can't be readonly.

In general, most parameters and many locals are never intended to be assigned to after they get their initial value. Allowing readonly on them would express that intent clearly.

One problem is that this feature might be an "attractive nuisance". Whereas the "right thing" to do would nearly always be to make parameters and locals readonly, it would clutter the code significantly to do so.

An idea to partly alleviate this is to allow the combination readonly var on a local variable to be contracted to val or something short like that. More generally we could try to simply think of a shorter keyword than the established readonly to express the readonly-ness.

Discussion continues in the C# Language Design repo. Vote to show your support. https://github.com/dotnet/csharplang/issues/188

Addressing Jared's answer, it would probably just have to be a compile-time feature - the compiler would prohibit you from writing to the variable after the initial declaration (which would have to include an assignment).

Can I see value in this? Potentially - but not a lot, to be honest. If you can't easily tell whether or not a variable is going to be assigned elsewhere in the method, then your method is too long.

For what it's worth, Java has this feature (using the final modifier) and I've very rarely seen it used other than in cases where it has to be used to allow the variable to be captured by an anonymous inner class - and where it is used, it gives me an impression of clutter rather than useful information.