Protobuf is not supporting text-format in the Google.Protobuf C# implementation at this moment. This project provides text format support for protobuf in C#.
PM> Install-Package Protobuf.Text
How to use:
// parse from text
TestAllTypes parsedFromText = TestAllTypes.Parser.ParseText(text);
// serialize to text
var text = parsedFromText.ToText();
// config
var config = File.ReadAllText("PATH");
var parsed = TextParser.Default.Parse<T>(config);
Related issue on Protobuf repo.