Fix "MMM.yy" date parsing in German culture. #114169

src/libraries/System.Private.CoreLib/src/System/Globalization/DateTimeParse.cs

@@ -3289,6 +3289,51 @@ private static bool ParseTimeZoneOffset(ref __DTString str, int len, scoped ref
             return true;
         }
 
+        /// Determines if a format string contains a day-of-month specifier ('d' or 'dd').
+        /// Properly handles quoted sections and escape characters.
+        private static bool FormatContainsDayOfMonthSpecifier(ReadOnlySpan<char> format)
+        {
+            if (format.IsEmpty)
+            {
+                return false;
+            }
+            bool inQuote = false;
+            for (int i = 0; i < format.Length; i++)
+            {
+                char ch = format[i];
+                // Skip the next character if it's escaped
+                if (ch == '\\' || ch == '%')
+                {
+                    i++;
+                    continue;
+                }
+                // Toggle quote state
+                if (ch == '\'' || ch == '"')
+                {
+                    inQuote = !inQuote;
+                    continue;
+                }
+                // Only check for 'd' when not in quotes
+                if (!inQuote && ch == 'd')
+                {
+                    // Make sure it's a day-of-month specifier (d or dd)
+                    // and not a day-of-week specifier (ddd or dddd)
+                    int repeatCount = 1;
+                    while (i + 1 < format.Length && format[i + 1] == 'd')
+                    {
+                        repeatCount++;
+                        i++;
+                    }
+                    // Only day-of-month specifiers (d or dd) trigger genitive case
+                    if (repeatCount <= 2)
+                    {
+                        return true;
+                    }
+                }
+            }
+            return false;
+        }
+
         /*=================================MatchAbbreviatedMonthName==================================
         **Action: Parse the abbreviated month name from string starting at str.Index.
         **Returns: A value from 1 to 12 for the first month to the twelfth month.
@@ -3297,7 +3342,7 @@ private static bool ParseTimeZoneOffset(ref __DTString str, int len, scoped ref
         **Exceptions: FormatException if an abbreviated month name can not be found.
         ==============================================================================*/
 
-        private static bool MatchAbbreviatedMonthName(ref __DTString str, DateTimeFormatInfo dtfi, scoped ref int result)
+        private static bool MatchAbbreviatedMonthName(ref __DTString str, DateTimeFormatInfo dtfi, scoped ref int result, ReadOnlySpan<char> format)
         {
             int maxMatchStrLen = 0;
             result = -1;
@@ -3357,12 +3402,11 @@ private static bool MatchAbbreviatedMonthName(ref __DTString str, DateTimeFormat
                     }
                 }
 
-                // Search genitive form.
-                if ((dtfi.FormatFlags & DateTimeFormatFlags.UseGenitiveMonth) != 0)
+                // Search genitive form only if the format contains a day-of-month specifier
+                if ((dtfi.FormatFlags & DateTimeFormatFlags.UseGenitiveMonth) != 0 && FormatContainsDayOfMonthSpecifier(format))
                 {
                     int tempResult = str.MatchLongestWords(dtfi.InternalGetGenitiveMonthNames(abbreviated: true), ref maxMatchStrLen);
-
-                    // We found a longer match in the genitive month name.  Use this as the result.
+                    // We found a longer match in the genitive month name. Use this as the result.
                     // tempResult + 1 should be the month value.
                     if (tempResult >= 0)
                     {
@@ -3399,7 +3443,7 @@ private static bool MatchAbbreviatedMonthName(ref __DTString str, DateTimeFormat
         **Exceptions: FormatException if a month name can not be found.
         ==============================================================================*/
 
-        private static bool MatchMonthName(ref __DTString str, DateTimeFormatInfo dtfi, scoped ref int result)
+        private static bool MatchMonthName(ref __DTString str, DateTimeFormatInfo dtfi, scoped ref int result, ReadOnlySpan<char> format)
         {
             int maxMatchStrLen = 0;
             result = -1;
@@ -3458,7 +3502,7 @@ private static bool MatchMonthName(ref __DTString str, DateTimeFormatInfo dtfi,
                 }
 
                 // Search genitive form.
-                if ((dtfi.FormatFlags & DateTimeFormatFlags.UseGenitiveMonth) != 0)
+                if ((dtfi.FormatFlags & DateTimeFormatFlags.UseGenitiveMonth) != 0 && FormatContainsDayOfMonthSpecifier(format))
                 {
                     int tempResult = str.MatchLongestWords(dtfi.InternalGetGenitiveMonthNames(abbreviated: false), ref maxMatchStrLen);
                     // We found a longer match in the genitive month name.  Use this as the result.
@@ -4045,15 +4089,15 @@ private static bool ParseByFormat(
                     {
                         if (tokenLen == 3)
                         {
-                            if (!MatchAbbreviatedMonthName(ref str, dtfi, ref tempMonth))
+                            if (!MatchAbbreviatedMonthName(ref str, dtfi, ref tempMonth, format.Value))
                             {
                                 result.SetBadDateTimeFailure();
                                 return false;
                             }
                         }
                         else
                         {
-                            if (!MatchMonthName(ref str, dtfi, ref tempMonth))
+                            if (!MatchMonthName(ref str, dtfi, ref tempMonth, format.Value))
                             {
                                 result.SetBadDateTimeFailure();
                                 return false;

src/libraries/System.Runtime/tests/System.Runtime.Tests/System/DateTimeTests.cs

@@ -14,6 +14,55 @@ namespace System.Tests
 {
     public class DateTimeTests
     {
+        [Fact]
+        public static void ParseExact_GenitiveMonthNames()
+        {
+            // Create a German culture with explicitly defined genitive month names
+            var culture = new CultureInfo("de-DE");
+            culture.DateTimeFormat.AbbreviatedMonthGenitiveNames = new[] { "Jan.", "Feb.", "März", "Apr.", "Mai", "Juni", "Juli", "Aug.", "Sept.", "Okt.", "Nov.", "Dez.", "" };
+            culture.DateTimeFormat.MonthGenitiveNames = new[] { "Januar", "Februar", "März", "April", "Mai", "Juni", "Juli", "August", "September", "Oktober", "November", "Dezember.", "" };
+            culture.DateTimeFormat.DayNames = new[] { "Sonntag", "Montag", "Dienstag", "Mittwoch", "Donnerstag", "Freitag", "Samstag" };
+            culture.DateTimeFormat.DateSeparator = ".";
+            // Regular month names (non-genitive)
+            culture.DateTimeFormat.AbbreviatedMonthNames = new[] { "Jan", "Feb", "Mär", "Apr", "Mai", "Jun", "Jul", "Aug", "Sep", "Okt", "Nov", "Dez", "" };
+            culture.DateTimeFormat.MonthNames = new[] { "Januar", "Februar", "März", "April", "Mai", "Juni", "Juli", "August", "September", "Oktober", "November", "Dezember", "" };
+            // Test cases for abbreviated month names (MMM)
+            // Case 1: Format without day specifier - should use regular month name
+            DateTime result;
+            bool success = DateTime.TryParseExact("Dez.20", "MMM.yy", culture, DateTimeStyles.None, out result);
+            Assert.True(success);
+            Assert.Equal(12, result.Month);
+            Assert.Equal(2020, result.Year);
+            // Case 2: Format with day-of-month specifier - should use genitive month name
+            success = DateTime.TryParseExact("Dez.20 01", "MMM.yy dd", culture, DateTimeStyles.None, out result);
+            Assert.False(success);
+            // Case 3: Format with day-of-month specifier before month - should use genitive month name
+            success = DateTime.TryParseExact("01 Dez.20", "d MMM.yy", culture, DateTimeStyles.None, out result);
+            Assert.False(success);
+            // Case 4: Format with day-of-week specifier - should use regular month name
+            success = DateTime.TryParseExact("Dienstag Dez.20", "dddd MMM.yy", culture, DateTimeStyles.None, out result);
+            Assert.True(success);
+            Assert.Equal(12, result.Month);
+            Assert.Equal(2020, result.Year);
+            // Test cases for full month names (MMMM)
+            // Case 5: Format without day specifier - should use regular month name
+            success = DateTime.TryParseExact("Dezember.20", "MMMM.yy", culture, DateTimeStyles.None, out result);
+            Assert.True(success);
+            Assert.Equal(12, result.Month);
+            Assert.Equal(2020, result.Year);
+            // Case 6: Format with day-of-month specifier - should use genitive month name
+            success = DateTime.TryParseExact("Dezember.20 01", "MMMM.yy dd", culture, DateTimeStyles.None, out result);
+            Assert.False(success);
+            // Case 7: Format with day-of-month specifier before month - should use genitive month name
+            success = DateTime.TryParseExact("01 Dezember.20", "d MMMM.yy", culture, DateTimeStyles.None, out result);
+            Assert.False(success);
+            // Case 8: Format with day-of-week specifier - should use regular month name
+            success = DateTime.TryParseExact("Dienstag Dezember.20", "dddd MMMM.yy", culture, DateTimeStyles.None, out result);
+            Assert.True(success);
+            Assert.Equal(12, result.Month);
+            Assert.Equal(2020, result.Year);
+        }
+
         [Fact]
         public static void MaxValue()
         {