In C# string how to replace two chararacters ignoring a number

Question

I am editing a serialized string becuase when deserilized it gives a parse error. so from a long Serialized string I want to edit "myVar\": \"0.2 mm" with "myVar\": \"0.2" if I use the follwoing code it works

string NewString = Serializedstring.Replace($"myVar\": \"0.2 mm", $"myVar\": \"0.2")

but my 0.2 is a varible that may change with every occurance. so all I want is to remove mm from the string "myVar\": \"0.2 mm"

Answer 1

If your JSON is coming in with two different formats, rather than trying to hack the JSON string into something usable, it is much safer to use a custom JsonConverter. For example:

public class MillimetreJsonConverter : JsonConverter<double>
{
    public override double Read(ref Utf8JsonReader reader, Type typeToConvert, 
        JsonSerializerOptions options)
    {
        // First try to get a double, if it works then simply return it
        if(reader.TryGetDouble(out var val))
        {
            return val;
        }
        
        // Otherwise we get the string value e.g. "0.2 mm" and
        // do some simple string manipulation on it
        var value = reader.GetString()!;
        value = value.Replace(" mm", "");
        
        if(double.TryParse(value, out var result))
        {
            return result;
        }

        // If we get here, perhaps we should throw an exception?
        return 0;
    }

    public override void Write(Utf8JsonWriter writer, double value, 
        JsonSerializerOptions options)
    {
        // You can fill this in if you need it
        throw new NotImplementedException();
    }
}

Now you can modify the class to deserialise into, assuming your JSON looks like this {"myVar": "0.2 mm"},:

public class Foo
{
    [JsonConverter(typeof(MillimetreJsonConverter))]
    public double myVar { get; set; }
}

Finally, it's simple to deserialise:

var foo = JsonSerializer.Deserialize<Foo>(json);