Non-read only alternative to anonymous types

Is there an alternative to anonymous types which will allow me to concisely define a simple "record" type like this without making it read-only?

No. You'll have to make a nominal type.

If no, is there something fundamentally wrong with wanting to do this?

No, it's a reasonable feature that we have considered before.

I note that in Visual Basic, anonymous types are mutable if you want them to be.

The only thing that is really "fundamentally wrong" about a mutable anonymous type is that it would be dangerous to use one as a hash key. We designed anonymous types with the assumptions that (1) you're going to use them as the keys in equijoins in LINQ query comprehensions, and (2) in LINQ-to-Objects and other implementations, joins will be implemented using hash tables. Therefore anonymous types should be useful as hash keys, and mutable hash keys are dangerous.

In Visual Basic, the GetHashCode implementation does not consume any information from mutable fields of anonymous types. Though that is a reasonable compromise, we simply decided that in C# the extra complexity wasn't worth the effort.

No, you'll have to create your own class or struct to do this (preferrably a class if you want it to be mutable - mutable structs are horrible).

If you don't care about Equals/ToString/GetHashCode implementations, that's pretty easy:

public class MyClass {
    public bool Foo { get; set; }
    public bool Bar { get; set; }
}

(I'd still use properties rather than fields, for various reasons.)

Personally I usually find myself wanting an immutable type which I can pass between methods etc - I want a named version of the existing anonymous type feature...