enhance invariant mode checks (#1917)

Author: Wraith2

src/Microsoft.Data.SqlClient/netcore/src/Microsoft/Data/SqlClient/SqlConnection.cs

@@ -1812,7 +1812,35 @@ private bool TryOpen(TaskCompletionSource<DbConnectionInternal> retry, SqlConnec
                 // are not present. Throwing on open with a meaningful message helps identify the issue.
                 if (_cultureCheckState == CultureCheckState.Unknown)
                 {
-                    _cultureCheckState = CultureInfo.GetCultureInfo("en-US").EnglishName.Contains("Invariant") ? CultureCheckState.Invariant : CultureCheckState.Standard;
+                    // check if invariant state has been set by appcontext switch directly 
+                    if (AppContext.TryGetSwitch("System.Globalization.Invariant", out bool isEnabled) && isEnabled)
+                    {
+                        _cultureCheckState = CultureCheckState.Invariant;
+                    }
+                    else
+                    {
+                        // check if invariant state has been set through environment variables
+                        string envValue = Environment.GetEnvironmentVariable("DOTNET_SYSTEM_GLOBALIZATION_INVARIANT");
+                        if (string.Equals(envValue, bool.TrueString, StringComparison.OrdinalIgnoreCase) || string.Equals(envValue, "1", StringComparison.OrdinalIgnoreCase))
+                        {
+                            _cultureCheckState = CultureCheckState.Invariant;
+                        }
+                        else
+                        {
+                            // if it hasn't been manually set it could still apply if the os doesn't have
+                            //  icu libs installed or is a native binary with icu support trimmed away
+                            // netcore 3.1 to net5 do not throw in attempting to create en-us in inariant mode
+                            // net6 and greater will throw so catch and infer invariant mode from the exception
+                            try
+                            {
+                                _cultureCheckState = CultureInfo.GetCultureInfo("en-US").EnglishName.Contains("Invariant") ? CultureCheckState.Invariant : CultureCheckState.Standard;
+                            }
+                            catch (CultureNotFoundException)
+                            {
+                                _cultureCheckState = CultureCheckState.Invariant;
+                            }
+                        }
+                    }
                 }
                 if (_cultureCheckState == CultureCheckState.Invariant)
                 {