Build query string for System.Net.HttpClient get

Question

If I wish to submit a http get request using System.Net.HttpClient there seems to be no api to add parameters, is this correct?

Yes.

Is there any simple api available to build the query string that doesn't involve building a name value collection and url encoding those and then finally concatenating them?

Sure:

var query = HttpUtility.ParseQueryString(string.Empty);
query["foo"] = "bar<>&-baz";
query["bar"] = "bazinga";
string queryString = query.ToString();

will give you the expected result:

foo=bar%3c%3e%26-baz&bar=bazinga

You might also find the UriBuilder class useful:

var builder = new UriBuilder("http://example.com");
builder.Port = -1;
var query = HttpUtility.ParseQueryString(builder.Query);
query["foo"] = "bar<>&-baz";
query["bar"] = "bazinga";
builder.Query = query.ToString();
string url = builder.ToString();

will give you the expected result:

http://example.com/?foo=bar%3c%3e%26-baz&bar=bazinga

that you could more than safely feed to your HttpClient.GetAsync method.

For those who do not want to include System.Web in projects that don't already use it, you can use FormUrlEncodedContent from System.Net.Http and do something like the following:

keyvaluepair version

string query;
using(var content = new FormUrlEncodedContent(new KeyValuePair<string, string>[]{
    new KeyValuePair<string, string>("ham", "Glazed?"),
    new KeyValuePair<string, string>("x-men", "Wolverine + Logan"),
    new KeyValuePair<string, string>("Time", DateTime.UtcNow.ToString()),
})) {
    query = content.ReadAsStringAsync().Result;
}

dictionary version

string query;
using(var content = new FormUrlEncodedContent(new Dictionary<string, string>()
{
    { "ham", "Glaced?"},
    { "x-men", "Wolverine + Logan"},
    { "Time", DateTime.UtcNow.ToString() },
})) {
    query = content.ReadAsStringAsync().Result;
}

In a ASP.NET Core project you can use the QueryHelpers class, available in the Microsoft.AspNetCore.WebUtilities namespace for ASP.NET Core, or the .NET Standard 2.0 NuGet package for other consumers:

// using Microsoft.AspNetCore.WebUtilities;
var query = new Dictionary<string, string>
{
    ["foo"] = "bar",
    ["foo2"] = "bar2",
    // ...
};

var response = await client.GetAsync(QueryHelpers.AddQueryString("/api/", query));

TL;DR: do not use accepted version as It's completely broken in relation to handling unicode characters, and never use internal API

I've actually found weird double encoding issue with the accepted solution:

So, If you're dealing with characters which need to be encoded, accepted solution leads to double encoding:

• query parameters are auto encoded by using NameValueCollection indexer (and this uses UrlEncodeUnicode, not regular expected UrlEncode(!))
• Then, when you call uriBuilder.Uri it creates new Uri using constructor which does encoding one more time (normal url encoding)
• That cannot be avoided by doing uriBuilder.ToString() (even though this returns correct Uri which IMO is at least inconsistency, maybe a bug, but that's another question) and then using HttpClient method accepting string - client still creates Uri out of your passed string like this: new Uri(uri, UriKind.RelativeOrAbsolute)

Small, but full repro:

var builder = new UriBuilder
{
    Scheme = Uri.UriSchemeHttps,
    Port = -1,
    Host = "127.0.0.1",
    Path = "app"
};

NameValueCollection query = HttpUtility.ParseQueryString(builder.Query);

query["cyrillic"] = "кирилиця";

builder.Query = query.ToString();
Console.WriteLine(builder.Query); //query with cyrillic stuff UrlEncodedUnicode, and that's not what you want

var uri = builder.Uri; // creates new Uri using constructor which does encode and messes cyrillic parameter even more
Console.WriteLine(uri);

// this is still wrong:
var stringUri = builder.ToString(); // returns more 'correct' (still `UrlEncodedUnicode`, but at least once, not twice)
new HttpClient().GetStringAsync(stringUri); // this creates Uri object out of 'stringUri' so we still end up sending double encoded cyrillic text to server. Ouch!

Output:

?cyrillic=%u043a%u0438%u0440%u0438%u043b%u0438%u0446%u044f

https://127.0.0.1/app?cyrillic=%25u043a%25u0438%25u0440%25u0438%25u043b%25u0438%25u0446%25u044f

As you may see, no matter if you do uribuilder.ToString() + httpClient.GetStringAsync(string) or uriBuilder.Uri + httpClient.GetStringAsync(Uri) you end up sending double encoded parameter

Fixed example could be:

var uri = new Uri(builder.ToString(), dontEscape: true);
new HttpClient().GetStringAsync(uri);

But this uses obsolete Uri constructor

P.S on my latest .NET on Windows Server, Uri constructor with bool doc comment says "obsolete, dontEscape is always false", but actually works as expected (skips escaping)

So It looks like another bug...

And even this is plain wrong - it send UrlEncodedUnicode to server, not just UrlEncoded what server expects

Update: one more thing is, NameValueCollection actually does UrlEncodeUnicode, which is not supposed to be used anymore and is incompatible with regular url.encode/decode (see NameValueCollection to URL Query?).

So the bottom line is: never use this hack with NameValueCollection query = HttpUtility.ParseQueryString(builder.Query); as it will mess your unicode query parameters. Just build query manually and assign it to UriBuilder.Query which will do necessary encoding and then get Uri using UriBuilder.Uri.

Prime example of hurting yourself by using code which is not supposed to be used like this

You might want to check out Flurl [disclosure: I'm the author], a fluent URL builder with optional companion lib that extends it into a full-blown REST client.

var result = await "https://api.com"
    // basic URL building:
    .AppendPathSegment("endpoint")
    .SetQueryParams(new {
        api_key = ConfigurationManager.AppSettings["SomeApiKey"],
        max_results = 20,
        q = "Don't worry, I'll get encoded!"
    })
    .SetQueryParams(myDictionary)
    .SetQueryParam("q", "overwrite q!")

    // extensions provided by Flurl.Http:
    .WithOAuthBearerToken("token")
    .GetJsonAsync<TResult>();

Check out the docs for more details. The full package is available on NuGet:

PM> Install-Package Flurl.Http

or just the stand-alone URL builder:

PM> Install-Package Flurl