Updated delimiter detection algorithm.

- Strips escaped text based on mode.
- Only looks for delimiters that appear on every line.
- Uses CultureInfo.TextInfo.ListSeparator if it's on every line.

src/CsvHelper/Configuration/IParserConfiguration.cs

@@ -14,6 +14,11 @@ namespace CsvHelper.Configuration
 	/// </summary>
 	public interface IParserConfiguration
 	{
+		/// <summary>
+		/// Gets the culture info used to read an write CSV files.
+		/// </summary>
+		CultureInfo CultureInfo { get; }
+
 		/// <summary>
 		/// Cache fields that are created when parsing.
 		/// Default is false.

src/CsvHelper/Configuration/IReaderConfiguration.cs

@@ -45,11 +45,6 @@ public interface IReaderConfiguration : IParserConfiguration
 		/// </summary>
 		ReadingExceptionOccurred ReadingExceptionOccurred { get; }
 
-		/// <summary>
-		/// Gets the culture info used to read an write CSV files.
-		/// </summary>
-		CultureInfo CultureInfo { get; }
-
 		/// <summary>
 		/// Prepares the header field for matching against a member name.
 		/// The header field and the member name are both ran through this function.

src/CsvHelper/CsvParser.cs

@@ -280,10 +280,22 @@ private void DetectDelimiter()
 		{
 			var text = new string(buffer, 0, charsRead);
 
+			if (mode == CsvMode.RFC4180)
+			{
+				// Remove text in between pairs of quotes.
+				text = Regex.Replace(text, $"({quote}.*?{quote})", string.Empty);
+			}
+			else if (mode == CsvMode.Escape)
+			{
+				// Remove escaped characters.
+				text = Regex.Replace(text, $"({escape}.)", string.Empty);
+			}
+
+			var lineDelimiterCounts = new List<Dictionary<string, int>>();
 			while (text.Length > 0)
 			{
+				// Since all escaped text has been removed, we can reliably read line by line.
 				var index = text.IndexOf(newLine);
-
 				var line = index > -1 ? text.Substring(0, index + newLine.Length) : text;
 
 				var delimiterCounts = new Dictionary<string, int>();
@@ -294,17 +306,50 @@ private void DetectDelimiter()
 					delimiterCounts[delimiter] = Regex.Matches(line, pattern).Count;
 				}
 
-				var maxCount = delimiterCounts.OrderByDescending(c => c.Value).First();
-				if (maxCount.Value > 0)
-				{
-					delimiter = maxCount.Key;
-					delimiterFirstChar = delimiter[0];
-					configuration.Validate();
+				lineDelimiterCounts.Add(delimiterCounts);
 
-					break;
+				text = index > -1 ? text.Substring(index + newLine.Length) : string.Empty;
+			}
+
+			if (lineDelimiterCounts.Count > 1)
+			{
+				// The last line isn't complete and can't be used to reliably detect a delimiter.
+				lineDelimiterCounts.Remove(lineDelimiterCounts.Last());
+			}
+
+			// Rank only the delimiters that appear on every line.
+			var delimiters =
+			(
+				from counts in lineDelimiterCounts
+				from count in counts
+				group count by count.Key into g
+				where g.All(x => x.Value > 0)
+				let sum = g.Sum(x => x.Value)
+				orderby sum descending
+				select new
+				{
+					Delimiter = g.Key,
+					Count = sum
 				}
+			).ToList();
 
-				text = index > -1 ? text.Substring(index + newLine.Length) : string.Empty;
+			string? newDelimiter = null;
+			if (delimiters.Any(x => x.Delimiter == configuration.CultureInfo.TextInfo.ListSeparator))
+			{
+				// The culture's separator is on every line. Assume this is the delimiter.
+				newDelimiter = configuration.CultureInfo.TextInfo.ListSeparator;
+			}
+			else
+			{
+				// Choose the highest ranked delimiter.
+				newDelimiter = delimiters.Select(x => x.Delimiter).FirstOrDefault();
+			}
+
+			if (newDelimiter != null)
+			{
+				delimiter = newDelimiter;
+				delimiterFirstChar = newDelimiter[0];
+				configuration.Validate();
 			}
 		}
 

tests/CsvHelper.Tests/Parsing/DetectDelimiterTests.cs

@@ -201,7 +201,7 @@ public void Parse_TextHasRegularCharDelimiter_DetectsDelimiter()
 		}
 
 		[Fact]
-		public void Parse_MultipleLines_DetectsDelimiterFromFirstLineOnly()
+		public void Parse_MultipleLines_DetectsDelimiterThatIsOnEveryLine()
 		{
 			var s = new StringBuilder();
 			s.Append("Id;Name\r\n");
@@ -241,5 +241,43 @@ public void Parse_NoDelimiter_DoesNotDetect()
 				Assert.Equal("`", parser.Delimiter);
 			}
 		}
+
+		[Fact]
+		public void Parse_CulturesSeparatorOccursLessButIsOnEveryLine_CulturesSeparatorIsDetected()
+		{
+			var s = new StringBuilder();
+			s.Append("1;2,3;4\r\n");
+			s.Append("5;6,7;8\r\n");
+			var config = new CsvConfiguration(CultureInfo.InvariantCulture)
+			{
+				DetectDelimiter = true,
+			};
+			using (var reader = new StringReader(s.ToString()))
+			using (var parser = new CsvParser(reader, config))
+			{
+				parser.Read();
+
+				Assert.Equal(",", parser.Delimiter);
+			}
+		}
+
+		[Fact]
+		public void Parse_CulturesSeparatorOccursLessAndIsNotOnEveryLine_CulturesSeparatorIsDetected()
+		{
+			var s = new StringBuilder();
+			s.Append("1;2;3;4\r\n");
+			s.Append("5;6,7;8\r\n");
+			var config = new CsvConfiguration(CultureInfo.InvariantCulture)
+			{
+				DetectDelimiter = true,
+			};
+			using (var reader = new StringReader(s.ToString()))
+			using (var parser = new CsvParser(reader, config))
+			{
+				parser.Read();
+
+				Assert.Equal(";", parser.Delimiter);
+			}
+		}
 	}
 }