class test
{
internal unsafe void ToLowerInvariant ()
{
string tmp = "fooo";
fixed (char* source = tmp); // problem, line 8
char* bar = tmp;
}
}

This produces the following error on MS:
test.cs(8,13): error CS0029: Cannot implicitly convert type 'string' to 'char*'

It appears to allow implicit conversions of some types (maybe just string) within the fixed initialization. It fails again when using it in the embedded staements of the fixed context.

I dont know if this code is valid, but mono compiles it.
testcase1.cs:8: incompatible types in assignment: no conversion from `System.String' to `char *'

class test
{
[NonSerialized] private char start_char;
internal unsafe void ToLowerInvariant ()
{
string tmp;
fixed (char* source = tmp); // problem, line 8
}
}