Twiddle Multiple Bits
The following is the IL generated for zerobits(uint x, int n, int i);

There seems to be full optimization by inlining sizeof(uint) * 8 = 32, and uint.MaxValue = -1 = 0xffffffff

The CIL runtime stack works only with 4 byte or 8 byte integers, even the LDC_I4_S pushes a byte as a 4 byte integer to the stack, so, there is no performance gain in changing parameter types to smaller integers, byte or short

A slight improvement (may be);

These methods need not to be instance members and can be defined static. Changing them to static will remove 1 push instruction to the stack (argument 0, the instance to invoke the method on) at the caller site.

That's all. 100 million iterations on my 2-cpu 8 core windows 7 HP machine executes in 17-18 ms, and 8-9 ms of it seems to be the iteration (empty loop) itself.

IL_0000:  ldarg.1       // push x (stack : x)
IL_0001:  ldc.i4.m1     // push 0xffffffff (uint.MaxValue is inlined, m1 = -1 = 0xffffffff) (stack : x, 0xffffffff)
IL_0002:  ldc.i4.s   32 // push 0x20 (sizeof(uint) * 8 is inlined) (stack: x, 0xffffffff, 0x20)
IL_0004:  ldarg.2       // push n (stack: x, 0xffffffff, 0x20, n)

IL_0005:  sub           // subtract -&gt; (0x20 - n) (stack : x, 0xffffffff, 0x20-n)

// The following two instructions are for ENSURING SHIFTING RIGHT WITH A POSITIVE INTEGER
IL_0006:  ldc.i4.s   31 // push 0x1f (stack : x, 0xffffffff, 0x20-n, 0x1f)
IL_0008:  and           // (0x20 - n) &amp; 0x1f (stack : x, 0xffffffff, 0x20-n) - We assume that n &lt; 0x20

IL_0009:  shr.un        // shift right unsigned, 0xffffffff &gt;&gt; (0x20-n) (stack : x, 0xffffffff &gt;&gt; (0x20-n))

IL_000a:  ldarg.3       // push i (stack : x, 0xffffffff &gt;&gt; (0x20-n), i)

// The following two instructions are for ENSURING SHIFTING LEFT WITH A POSITIVE INTEGER
IL_000b:  ldc.i4.s   31 // push 0x1f (stack : x, 0xffffffff &gt;&gt; (0x20-n), i, 0x1f)
IL_000d:  and           // (i &amp; 0x1f) (stack : x, 0xffffffff &gt;&gt; (0x20-n), i) - We assume that i &lt; 0x20

IL_000e:  shl           // shift left (0x20-n) &lt;&lt; i (stack : x, (0xffffffff &gt;&gt; (0x20-n)) &lt;&lt; i)
IL_000f:  not           // not (stack : x, ~(0xffffffff &gt;&gt; (0x20-n)) &lt;&lt; i))
IL_0010:  and           // and x &amp; ~(0xffffffff &gt;&gt; (0x20-n)) &lt;&lt; i) (stack : x &amp; ~((0xffffffff &gt;&gt; (0x20-n)) &lt;&lt; i)
IL_0011:  ret           // return x &amp; ~(0xffffffff &gt;&gt; (0x20-n)) &lt;&lt; i)