Sending query strings using HttpClient in C#

When using HttpClient, it handles the encoding of the URI internally using the Uri class, so you don’t need to encode the query string manually.

For example, sending a request with a query string:

await httpClient.GetAsync("https://localhost:5000/movies/search?title=Inception&genre=Science Fiction");

This results in the following encoded request:

GET https://localhost:5000/movies/search?title=Inception&genre=Science%20Fiction HTTP/1.1

In this case, "Inception" remains unchanged, but "Science Fiction" gets encoded as Science%20Fiction, where the space is replaced with %20.

Building a query string with QueryHelpers.AddQueryString()

You can build the query string dynamically using QueryHelpers.AddQueryString() from Microsoft.AspNetCore.WebUtilities.

using Microsoft.AspNetCore.WebUtilities;

var query = new Dictionary<string, string>
{
    ["title"] = "The Matrix",
    ["director"] = "Wachowski"
};

var uri = QueryHelpers.AddQueryString("https://localhost:5000/movies/search", query);

var result = await httpClient.GetAsync(uri);

This will generate the following request:

GET https://localhost:5000/movies/search?title=The%20Matrix&director=Wachowski HTTP/1.1

If you prefer not to use QueryHelpers or want more control over how the query string is built, you can create your own query string builder.

public static class RequestUriUtil
{
    public static string GetUriWithQueryString(string requestUri, Dictionary<string, string> queryStringParams)
    {
        var sb = new StringBuilder();
        sb.Append(requestUri);

        bool firstParam = true;
        foreach (var param in queryStringParams)
        {
            if (param.Value != null)
            {
                sb.Append(firstParam ? "?" : "&");
                sb.Append(param.Key);
                sb.Append("=");
                sb.Append(param.Value);
                firstParam = false;
            }
        }
        return sb.ToString();
    }
}

Usage:

var query = new Dictionary<string, string>
{
    ["title"] = "Blade Runner",
    ["year"] = "1982"
};

var requestUriWithQuery = RequestUriUtil.GetUriWithQueryString("https://localhost:5000/movies/search", query);

var result = await httpClient.GetAsync(requestUriWithQuery);

This generates:

GET https://localhost:5000/movies/search?title=Blade%20Runner&year=1982 HTTP/1.1

HttpClient Extension Method for Query Strings

You can simplify using query strings with an extension method on HttpClient:

public static class HttpClientExtensions
{
    public static async Task<HttpResponseMessage> GetWithQueryStringAsync(this HttpClient client, string uri, Dictionary<string, string> queryStringParams)
    {
        var url = QueryHelpers.AddQueryString(uri, queryStringParams);
        return await client.GetAsync(url);
    }
}

Usage:

var query = new Dictionary<string, string>
{
    ["title"] = "Interstellar",
    ["director"] = "Nolan"
};

var result = await httpClient.GetWithQueryStringAsync("https://localhost:5000/movies/search", query);

This will send the following request:

GET https://localhost:5000/movies/search?title=Interstellar&director=Nolan HTTP/1.1

This method helps encapsulate the query string construction logic and makes the calling code more concise.