Fix LocalAppContextSwitches race conditions in tests

src/Microsoft.Data.SqlClient/tests/Common/Common.csproj

@@ -8,6 +8,7 @@
     <IntermediateOutputPath>$(ObjFolder)$(Configuration).$(Platform).$(AssemblyName)</IntermediateOutputPath>
     <OutputPath>$(BinFolder)$(Configuration).$(Platform).$(AssemblyName)</OutputPath>
     <IsTestProject>true</IsTestProject>
+    <Nullable>enable</Nullable>
   </PropertyGroup>
 
   <!-- OS Constants ==================================================== -->

src/Microsoft.Data.SqlClient/tests/Common/Fixtures/CertificateFixtureBase.cs

@@ -67,7 +67,7 @@ protected X509Certificate2 CreateCertificate(string subjectName, IEnumerable<str
         DateTimeOffset notBefore = DateTimeOffset.UtcNow.AddDays(-1);
         DateTimeOffset notAfter = DateTimeOffset.UtcNow.AddDays(1);
         byte[] passwordBytes = new byte[32];
-        string password = null;
+        string? password = null;
         Random rnd = new Random();
 
         rnd.NextBytes(passwordBytes);
@@ -256,7 +256,7 @@ private static RSA CreateRSA(bool forceCsp)
 
     protected void AddToStore(X509Certificate2 cert, StoreLocation storeLocation, StoreName storeName)
     {
-        CertificateStoreContext storeContext = _certificateStoreModifications.Find(csc => csc.Location == storeLocation && csc.Name == storeName);
+        CertificateStoreContext? storeContext = _certificateStoreModifications.Find(csc => csc.Location == storeLocation && csc.Name == storeName);
 
         if (storeContext == null)
         {

src/Microsoft.Data.SqlClient/tests/Common/Fixtures/ColumnEncryptionCertificateFixture.cs

@@ -25,7 +25,7 @@ public sealed class ColumnEncryptionCertificateFixture : CertificateFixtureBase
     public X509Certificate2 CertificateWithoutPrivateKey { get; }
 
     private readonly X509Certificate2 _currentUserCertificate;
-    private readonly X509Certificate2 _localMachineCertificate;
+    private readonly X509Certificate2? _localMachineCertificate;
 
     public ColumnEncryptionCertificateFixture()
     {
@@ -57,11 +57,19 @@ public ColumnEncryptionCertificateFixture()
 
     public X509Certificate2 GetCertificate(StoreLocation storeLocation)
     {
-        return storeLocation == StoreLocation.CurrentUser
-            ? _currentUserCertificate
-            : storeLocation == StoreLocation.LocalMachine && IsAdmin
-                ? _localMachineCertificate
-                : throw new InvalidOperationException("Attempted to retrieve the certificate added to the local machine store; this requires administrator rights.");
+        if (storeLocation == StoreLocation.CurrentUser)
+        {
+            return _currentUserCertificate;
+        }
+
+        if (storeLocation == StoreLocation.LocalMachine &&
+            IsAdmin &&
+            _localMachineCertificate is not null)
+        {
+            return _localMachineCertificate;
+        }
+
+        throw new InvalidOperationException("Attempted to retrieve the certificate added to the local machine store; this requires administrator rights.");
     }
 
     public static bool IsAdmin

src/Microsoft.Data.SqlClient/tests/Common/Fixtures/ColumnMasterKeyCertificateFixture.cs

@@ -21,7 +21,7 @@ public ColumnMasterKeyCertificateFixture()
     {
     }
 
-    public X509Certificate2 ColumnMasterKeyCertificate { get; }
+    public X509Certificate2? ColumnMasterKeyCertificate { get; }
 
     protected ColumnMasterKeyCertificateFixture(bool createCertificate)
     {

src/Microsoft.Data.SqlClient/tests/Common/Fixtures/CspCertificateFixture.cs

@@ -32,17 +32,17 @@ public CspCertificateFixture()
 
     public string CspCertificatePath { get; }
 
-    public string CspKeyPath { get; }
+    public string? CspKeyPath { get; }
 
-    private string GetCspPathFromCertificate()
+    private string? GetCspPathFromCertificate()
     {
-        RSA privateKey = CspCertificate.GetRSAPrivateKey();
+        RSA? privateKey = CspCertificate.GetRSAPrivateKey();
 
         if (privateKey is RSACryptoServiceProvider csp)
         {
             return string.Concat(csp.CspKeyContainerInfo.ProviderName, @"/", csp.CspKeyContainerInfo.KeyContainerName);
         }
-        else if (privateKey is RSACng cng)
+        else if (privateKey is RSACng cng && cng.Key.Provider is not null)
         {
             return string.Concat(cng.Key.Provider.Provider, @"/", cng.Key.KeyName);
         }