One of the first things you’ll notice in Go is that two different types are commonly used for representing text. string and []byte. A quick example is the regexp package which has functions for both string and []byte.

What is the difference?

string is immutable and []byte is mutable. Both can contain arbitrary bytes.

The name “string” implies unicode text but this is not enforced. Operating on string is like operating on []byte. You are working with bytes not characters.

They are nearly identical and differ only in mutability. The strings and bytes packages are nearly identical apart from the type that they use.

Q: If strings are just arbitrary bytes, then how do you work with characters?

A: What you are thinking of as a character, Go calls a rune. One way to iterate the characters in a string is to use the for...range loop. Range will parse the string as UTF-8 and iterate the runes. Read the for loop section of Effective Go for more information.

When to use string?

Ask not when to use string but rather, when to use []byte. Always start with string and switch to []byte when justified.

When to use []byte?

Use []byte when you need to make many changes to a string. Since string is immutable, any change will allocate a new string. You can get better performance by using []byte and avoiding the allocations.

C# perspective: []byte is to System.StringBuilder as string is to System.String when it comes to performance.

Even if your code isn’t directly manipulating the string, you may want to use []byte if you are using packages which require it so you can avoid the conversion.

Converting to and from []byte is easy. Just remember that each conversion creates a copy of the value.

s := "some string"
b := []byte(s) // convert string -> []byte
s2 := string(b) // convert []byte -> string

Converting to/from string and []byte copies the entire value. Using lots of type conversions in your code is typically a warning sign that you need to reevaluate the types you are using. You want to minimize conversions both for performance and clean code.

More about strings

The Go blog has posted in detail about strings, bytes, runes, and characters in Go. You should definitely read that post to fully understand the topic.

Update: Thanks to @mholt6 for reviewing the post and helping improve it!