如何存储可空值并遍历它们？

Question

Thanks for taking a moment. I have a url . I'm trying to build the querystring parameters from these nullable input parameters you see below. I'm thinking what I need to do is store the nullable parameters into some type of generic list and then if the given value of the parameter is not null, append it to the url. So for example, If they supply the string? state a value of 'OH' my url would look like ( http://www.thiscoolsiteJas.com/api/v1/moratorium?state=OH ). How do I do this ? Any advice or direction would be greatly appreciated. - Jason

 public  GetMoratoriums(string? state, int? system, DateTime? inEffectOn, bool? expired, int? reason) {
       //How do I build this with parameters  using the input parameters??
        string url = http://www.thiscoolsiteJas.com/api/v1/moratorium?";

}

Answer 1

Something like this?

var queryStrings = new List<string>();

if (state.HasValue) { queryStrings.Add($"state={state.Value}"); }
if (system.HasValue) { queryStrings.Add($"system={system.Value}"); }
if (inEffectOn.HasValue) { queryStrings.Add($"inEffectOn={inEffectOn.Value}"); }
if (expired.HasValue) { queryStrings.Add($"expired={expired.Value}"); }
if (reason.HasValue) { queryStrings.Add($"reason={reason.Value}"); }

var result = @"http://www.thiscoolsiteJas.com/api/v1/moratorium";
if (queryStrings.Any()) { result += $"?{String.Join("&", queryStrings)}"; }
return result;

Answer 2

I'm sure there is a better way to do it but this is how I would handle it.

string url = http://www.thiscoolsiteJas.com/api/v1/moratorium?";

var queryStringParams = new List<KeyValuePair<string, string>>();

if (!string.IsNullOrEmpty(state))
   {
       queryStringParams.Add("state",state);
   }
etc...

var newUrl = new Uri(QueryHelpers.AddQueryString(url, queryStringParams));

Answer 3

Object oriented approach ;).

public class Parameter
{
    private readonly string _name;

    private readonly object _value;

    public Parameter(string name, object value)
    {
        _name = name;
        _value = value;
    }

    public string ForQuery()
    {
        return _value == null ? null : $"{_name}={_value}";
    }
}

Usage

string? state = "new_state";
int? system = 1;
DateTime? inEffectOn = default;
bool? expired = default;

var parameters = new[]
{
    new QueryParameter("state", state),
    new QueryParameter("system", system),
    new QueryParameter("inEffectOn", inEffectOn),
    new QueryParameter("expired", expired),
};

var queryParameters = parameters.Select(p => p.ForQuery()).Where(p => p != null);
var query = string.Join("&", queryParameters);

For example instead of passing long list of parameters you can pass a collection of value of type QueryParameter and build query.

With extension method

public static string AppendTo(this IEnumerable<QueryParameter> parameters, string baseUrl) 
{
    return parameters
        .Select(p => p.ForQuery())
        .Where(p => p != null)
        .Aggregate(
            new StringBuilder(baseUrl),
            (builder, p) => builder.Append(p).Append("&"),
            (builder) => 
            {
                if (builder.Length > 0)
                {
                    builder.Insert(0, "?");
                    builder.Length -= 1; // remove last "&"
                }
            })
        .ToString();
}

Usage

var parameters = new[]
{
    new QueryParameter("state", "new"),
    new QueryParameter("system", null),
    new QueryParameter("inEffectOn", null),
    new QueryParameter("expired", false),
};

var url = parameters.AppendTo("http://www.thiscoolsiteJas.com/api/v1/moratorium");