Optional Parameters in Web Api Attribute Routing

Another info: If you want use a Route Constraint, imagine that you want force that parameter has int datatype, then you need use this syntax:

[Route("v1/location/**{deviceOrAppid:int?}**", Name = "AddNewLocation")]

The ? character is put always before the last } character

For more information see: Optional URI Parameters and Default Values

An additional fact to complement @Kiran Chala's answer -

When we mark any parameter (appid) as optional in the action URI using ? character(for nullable value types) then we must provide default value to the parameter in the method signature as shown below:

[Route("v1/location/{deviceid}/{appid}", Name = "AddNewLocation")]
public location_fromuser Post(string deviceid, int? appid = null)

Ok, I fallen here with my internet research and I continue my way, because the accepted solution not working with dotnet core 3.1. So here is my solution, following this doc

[HttpPost]
[Route("{name}")]
[Route("{name}/parent/{parentId}")]
public async Task<IActionResult> PostSomething(string name, Guid? parentId = null)
{
    return Ok(await Task.FromResult(new List<string>()));
}

By this way many routes go to this single API function

For an incoming request like /v1/location/1234, as you can imagine it would be difficult for Web API to automatically figure out if the value of the segment corresponding to '1234' is related to appid and not to deviceid.

I think you should change your route template to be like [Route("v1/location/{deviceOrAppid?}", Name = "AddNewLocation")] and then parse the deiveOrAppid to figure out the type of id.

Also you need to make the segments in the route template itself optional otherwise the segments are considered as required. Note the ? character in this case. For example: [Route("v1/location/{deviceOrAppid?}", Name = "AddNewLocation")]