Merged PR 42182: Use explicit full-path for loading MsQuic.dll

Port of https://dev.azure.com/dnceng/internal/_git/dotnet-runtime/pullrequest/42181 for 8.0

# Description

Use explicit full-path for loading MsQuic.dll - it prevents accidental load of MsQuic.dll from another unintended directory (e.g. CWD = Current Working Directory) if the bundled MsQuic.dll fails to load (which can happen e.g. on WinServer 2012, where MsQuic is not supported).

# Customer Impact

Defense-in-depth

# Regression

No

# Testing

Targeted manual test on affected OS (Windows Server 2012), including self-contained and single-file publish scenarios.

# Risk

Low, changes are contained and have been thoroughly tested manually and in CI suite.

Author: rzikm

src/libraries/System.Net.Quic/src/System/Net/Quic/Internal/MsQuicApi.cs

@@ -83,8 +83,23 @@ static MsQuicApi()
 
         if (OperatingSystem.IsWindows())
         {
-            // Windows ships msquic in the assembly directory.
-            loaded = NativeLibrary.TryLoad(Interop.Libraries.MsQuic, typeof(MsQuicApi).Assembly, DllImportSearchPath.AssemblyDirectory, out msQuicHandle);
+#pragma warning disable IL3000 // Avoid accessing Assembly file path when publishing as a single file
+            // Windows ships msquic in the assembly directory next to System.Net.Quic, so load that.
+            // For single-file deployments, the assembly location is an empty string so we fall back
+            // to AppContext.BaseDirectory which is the directory containing the single-file executable.
+            string path = typeof(MsQuicApi).Assembly.Location is string assemblyLocation && !string.IsNullOrEmpty(assemblyLocation)
+                ? System.IO.Path.GetDirectoryName(assemblyLocation)!
+                : AppContext.BaseDirectory;
+#pragma warning restore IL3000
+
+            path = System.IO.Path.Combine(path, Interop.Libraries.MsQuic);
+
+            if (NetEventSource.Log.IsEnabled())
+            {
+                NetEventSource.Info(null, $"Attempting to load MsQuic from {path}");
+            }
+
+            loaded = NativeLibrary.TryLoad(path, typeof(MsQuicApi).Assembly, DllImportSearchPath.LegacyBehavior, out msQuicHandle);
         }
         else
         {
@@ -154,7 +169,7 @@ static MsQuicApi()
 
             if (version < s_minMsQuicVersion)
             {
-                NotSupportedReason =  $"Incompatible MsQuic library version '{version}', expecting higher than '{s_minMsQuicVersion}'.";
+                NotSupportedReason = $"Incompatible MsQuic library version '{version}', expecting higher than '{s_minMsQuicVersion}'.";
                 if (NetEventSource.Log.IsEnabled())
                 {
                     NetEventSource.Info(null, NotSupportedReason);
@@ -178,7 +193,7 @@ static MsQuicApi()
                 // Implies windows platform, check TLS1.3 availability
                 if (!IsWindowsVersionSupported())
                 {
-                    NotSupportedReason =  $"Current Windows version ({Environment.OSVersion}) is not supported by QUIC. Minimal supported version is {s_minWindowsVersion}.";
+                    NotSupportedReason = $"Current Windows version ({Environment.OSVersion}) is not supported by QUIC. Minimal supported version is {s_minWindowsVersion}.";
                     if (NetEventSource.Log.IsEnabled())
                     {
                         NetEventSource.Info(null, NotSupportedReason);