What is the difference between type conversion and type assertion?

What is the main differences between :

1. v = t.(aType) // type assertion
2. v = aType(t) // type conversion

Where should I use type assertion or type conversion ?

A type assertion asserts that t (an interface type) actually is a aType and t will be an aType; namely the one wrapped in the t interface. E.g. if you know that your var reader io.Reader actually is a *bytes.Buffer you can do var br *bytes.Buffer = reader.(*bytes.Buffer).

A type conversion converts one (non-interface) type to another, e.g. a var x uint8 to and int64 like var id int64 = int64(x).

Rule of thumb: If you have to wrap your concrete type into an interface and want your concrete type back, use a type assertion (or type switch). If you need to convert one concrete type to an other, use a type conversion.

tl;dr x.(T) asserts that the dynamic value of interface x is T at run time; T(x) converts the type of an expression x to some other type.

Type Assertion

You know that in Go an interface is basically a method set specification, and you can assign to an interface variable any value whose type implements that method set¹.

The type assertion written x.(T) asserts that the value stored in the interface x is of type T. You use a type assertion when you want to unbox that value.

One of the most common uses is when you have interface{} and you need to retrieve the concrete value it stores. A typical example, Context values:

func foo(ctx context.Context) {
    s := ctx.Value("my_key").(string) // signature is `Value(key interface{}) interface{}`
    // do something with s...
}

It is called assertion because at compile time it is not known whether x actually holds the concrete type T, but you assert that it does.
That's why the unchecked assertion y := x.(T) panics if x doesn't actually hold a T — you must use the comma-ok assignment v, ok := x.(T) to avoid it.

ctx = context.WithValue(ctx, "my_key", "foo")
s := ctx.Value("my_key").(int) // panic

v, ok := ctx.Value("my_key").(string)
fmt.Println(v, ok) // "foo" true

In addition, when T in x.(T) is an interface itself, the assertion checks that the value stored in x implements T. The outcome is the same as above.

Type Conversion

A type conversion written as T(x) instead "changes the type of an expression to the type specified by the conversion", i.e. changes the type of x to T. An important property of conversions is that they are statically checked². An invalid conversion simply won't compile:

	type Foo string
	type Bar int

	a := "foo"
	fmt.Println(Bar(a)) // cannot convert a (type string) to type Bar

The main condition for a conversion to be valid is assignability between the types involved, but there's several more, including conversions between numerical types, strings and byte/rune slices, directed channels, slices and array pointers, etc.

In simple terms, you use a conversion when you already know what are the types involved, and simply want to change one to the other:

b := []byte("foo")  // converts string literal to byte slice

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Notes:

1: more formally, when the value's method set is a superset of the interface method set; this is also why the empty interface interface{} can hold any value, because any set is a superset of an empty set.

2: type assertions are also checked at compile time when the type T in x.(T) does not implement the interface. In practice, this won't help you catch errors when x is interface{} since all types implement it.