How should I pass multiple parameters to an ASP.Net Web API GET?

I think the easiest way is to simply use AttributeRouting.

It's obvious within your controller, why would you want this in your Global WebApiConfig file?

Example:

    [Route("api/YOURCONTROLLER/{paramOne}/{paramTwo}")]
    public string Get(int paramOne, int paramTwo)
    {
        return "The [Route] with multiple params worked";
    }

The {} names need to match your parameters.

Simple as that, now you have a separate GET that handles multiple params in this instance.

Just add a new route to the WebApiConfig entries.

For instance, to call:

public IEnumerable<SampleObject> Get(int pageNumber, int pageSize) { ..

add:

config.Routes.MapHttpRoute(
    name: "GetPagedData",
    routeTemplate: "api/{controller}/{pageNumber}/{pageSize}"
);

Then add the parameters to the HTTP call:

GET //<service address>/Api/Data/2/10 

I just had to implement a RESTfull api where I need to pass parameters. I did this by passing the parameters in the query string in the same style as described by Mark's first example "api/controller?start=date1&end=date2"

In the controller I used a tip from URL split? in C#?

// uri: /api/courses
public IEnumerable<Course> Get()
{
    NameValueCollection nvc = HttpUtility.ParseQueryString(Request.RequestUri.Query);
    var system = nvc["System"];
    // BL comes here
    return _courses;
}

In my case I was calling the WebApi via Ajax looking like:

$.ajax({
        url: '/api/DbMetaData',
        type: 'GET',
        data: { system : 'My System',
                searchString: '123' },
        dataType: 'json',
        success: function (data) {
                  $.each(data, function (index, v) {
                  alert(index + ': ' + v.name);
                  });
         },
         statusCode: {
                  404: function () {
                       alert('Failed');
                       }
        }
   });

I hope this helps...

I found exellent solution on http://habrahabr.ru/post/164945/

public class ResourceQuery
{
   public string Param1 { get; set; }
   public int OptionalParam2 { get; set; }
}

public class SampleResourceController : ApiController
{
    public SampleResourceModel Get([FromUri] ResourceQuery query)
    {
        // action
    }
}

Using GET or POST is clearly explained by @LukLed. Regarding the ways you can pass the parameters I would suggest going with the second approach (I don't know much about ODATA either).

1.Serializing the params into one single JSON string and picking it apart in the API. http://forums.asp.net/t/1807316.aspx/1

This is not user friendly and SEO friendly

2.Pass the params in the query string. What is best way to pass multiple query parameters to a restful api?

This is the usual preferable approach.

3.Defining the params in the route: api/controller/date1/date2

This is definitely not a good approach. This makes feel some one date2 is a sub resource of date1 and that is not the case. Both the date1 and date2 are query parameters and comes in the same level.

In simple case I would suggest an URI like this,

api/controller?start=date1&end=date2

But I personally like the below URI pattern but in this case we have to write some custom code to map the parameters.

api/controller/date1,date2

Use Parameter Binding as describe completely here : http://www.asp.net/web-api/overview/formats-and-model-binding/parameter-binding-in-aspnet-web-api