Build a query string: retarget net10.0, safe UriBuilder helper, CS8620 fix

[vladimir-pecanac-main]

Updates the dotnet-querystrings/BuildQueryString sample for the republish of How to Build a Query String for a URL in C#.

Retarget

net7.0 -> net10.0 on all three projects (BuildQueryString, BuildQueryStringTests, BooksAPI). Package versions taken from the NuGet flat-container index on 2026-08-30, not from memory:

Package	Was	Now
Microsoft.Extensions.DependencyInjection	7.0.0	10.0.11
Microsoft.NET.Test.Sdk	17.5.0	18.9.0
MSTest.TestAdapter / MSTest.TestFramework	2.2.10	4.3.3
Moq	4.20.69	4.20.72
coverlet.collector	3.2.0	10.0.1
Swashbuckle.AspNetCore	6.2.3	10.2.3

The MSTest 2.x -> 4.x jump needed no source changes: the classic Assert.AreEqual / TestClass / TestMethod API compiles as-is and no MSTEST analyzer diagnostic fires in this folder.

Safe UriBuilder composition (new helper + test)

The article's central claim about UriBuilder was wrong: it escapes characters that are illegal anywhere in a URI, but it does not escape the query delimiters &, = and # inside a value. Assigning author=Tom & Jerry to Query yields ?author=Tom%20&%20Jerry, which a receiver parses as two parameters, the second named Jerry.

BuildUrlWithQueryStringUsingUriBuilderSafely is the one-line composition that does encode: build the query with QueryString.Create and hand the finished string to UriBuilder. The new test asserts the full round trip over Tom & Jerry and C#:

https://localhost:7220/api/Books?author=Tom%20%26%20Jerry&tag=C%23

parsed back to author = Tom & Jerry and tag = C#.

CS8620

BuildUrlWithQueryStringUsingAddQueryStringMethod and BuildUrlWithQueryStringUsingCreateMethod take Dictionary<string, string?>, and every caller declared Dictionary<string, string>, which warns:

warning CS8620: Argument of type 'Dictionary<string, string>' cannot be used for parameter 'queryParams'
of type 'Dictionary<string, string?>' ... due to differences in the nullability of reference types

Fixed at all four call sites (two in BooksApiService, two in QueryStringHelperUnitTests). The build is now warning-clean apart from NU1510 on the DI package, which the article tells the reader to install explicitly.

Idiom

BuildQueryString/Program.cs lifted to top-level statements. BooksAPI/Program.cs was read and already uses minimal WebApplication hosting with no Startup.cs, so it is untouched.

Build and tests

dotnet build BuildQueryString.sln -c Release   ->  Build succeeded, 0 errors
dotnet test  BuildQueryString.sln -c Release   ->  Passed! - Failed: 0, Passed: 14, Skipped: 0, Total: 14

13 pre-existing tests plus the one added. Every comparison-table cell in the article was re-produced by running the retargeted solution itself on .NET 10.0.10, not by reading reference pages.