How to get a Protobuf enum value from its string representation? - go

Question

I can get the string value of a Protobuf enum with this instruction:
str := testPB.Status_ENABLED.String()
How can I perform the inverse operation? (from a string, get the enum element).

Answer 1

The generated code has a map called <EnumName>_value of type map[string]int32. Then you can convert the numerical value to the actual defined type:
num := testPB.Status_value[str]
v := testPB.Status(num)
Consider that if the str value doesn't exist in the map (note that it's case sensitive), the map look-up will return 0. Depending on how you defined your protobuffer, the 0 value might be mapped to a an enum instance that does not have "zero" semantics. This is why it is recommended to map 0 to an "unknown" instance:
enum Status {
UNKNOWN = 0;
ENABLED = 1;
// and so on
}
Which in Go correctly yields a makeshift zero-value if the string representation is effectively unknown:
v := testPB.Status(testPB.Status_value["does_not_exist"])
fmt.Println(v == testPB.Status_UNKNOWN) // true
Go 1.18
With generics, it is possible to write reusable code to construct protobuffer enums from string values:
func Enum[T ~string, PB ~int32](val T, pbmap map[string]int32, dft PB) PB {
v, ok := pbmap[string(val)]
if !ok {
return dft
}
return PB(v)
}
where:
the type parameter T is the string representation, which could also be a type with underlying type string, e.g. type MyEnum string
the argument pbmap is the <EnumName>_value from the generated protobuffer code
the type parameter PB is the protobuffer enum type.
The function above takes a default value of type PB to (obviously) have a fallback in case the string representation is invalid, and also to allow type inference on PB, which otherwise would be used only as a return type.
Usage:
type SomeEnum string
const (
SomeEnumFoo SomeEnum = "FOO"
SomeEnumBar SomeEnum = "BAR"
)
func main() {
foo := SomeEnumFoo
v := Enum(foo, pb.SomeEnum_value, pb.SomeEnum_Foo)
// ^ T ^ map[string]int32 ^ default PB value
// v is type pb.SomeEnum
}