How to return multiple parameters using web method (web service) in C#?

The recommended method is to create custom data type class and return data as the class type instead of string.

Like this suppose you have this class

[Serializable]
public sealed class MyDataType
{
    public string Data{ get; set; }
    public string Data1 { get; set; }
}

then your web method in web service should be like

[WebMethod(EnableSession = true)]
public MyDataType TestMethod(string testId)
{
  //Some Code

    
    string data = string.Empty;
    string data1 = string.Empty;
    List<test1> datalist1 = new List<test1>();
    List<test> datalist = new List<test>();

    //coding
    data = jsonSerialize.Serialize(datalist1);
    data1 = jsonSerialize.Serialize(datalist);
    return new MyData { Data = data, Data1 = data1 };
}

Another method, not a good way but you could return an array of strings:

public string[] WebServiceMethod(...)
{
    return new string[] { data, data1 };
}

You'd then need to perform each bit of JSON deserialization separately on the client, which isn't a pleasant task

Or third solution is to check this link

You can simply return multiple values in web-api in C# as below

 [HttpGet]
 [Route("api/getexample")]
 public IHttpActionResult getAll()
 {
    
   //some code here
    
    return Ok(new { data1List, data2List, someValue });
 }

That should work.