Why is a boolean 4 bytes in .NET?

A bool is actually only 1 byte, but alignment may cause 4 bytes to be used on a 32-bit platform, or even 8 bytes on a 64-bit platform. For example, the Nullable<bool> (aka bool?) type uses a full 32 or 64 bits—depending on platform—even though it's comprised of just two bools. EDIT: As pointed out by Jon Skeet, padding for alignment isn't always present. As an example, an array of Nullable<bool>s takes only 2 bytes per object instead of 4 or 8.

But even 8 bits to represent a bool can be considered wasteful if you have many of them to store. For this reason, if you create a type that has many bools as members, (or uses many Nullable<> types), and users of your class might create many instances of it, you might consider using a BitVector32 instead. The framework itself uses this technique to reduce the memory footprint of many of the Windows Forms controls, for instance.

Because it's fast.

A 32-bit processor typically works with 32-bit values. Working with smaller values involves longer instructions, or extra logic.

The first result on a Google search for System.Boolean size told me that it's to do with memory alignment. It's faster to push around a four-byte Int32 than it is to work with individual bytes/bits.

I think it's only for performance, 32 bit values are much more efficient to manipulate.

Where'd you get that? System.Boolean takes only 1 byte.

Just try:

Console.WriteLine( sizeof( System.Boolean ).ToString() );