Changed delimiter detection to look line by line instead of the full buffer.

Author: JoshClose

src/CsvHelper/CsvParser.cs

@@ -264,22 +264,34 @@ public async Task<bool> ReadAsync()
 		}
 
 		private void DetectDelimiter()
-		{
+		{			
 			var text = new string(buffer, 0, charsRead);
-			var delimiterCounts = new Dictionary<string, int>();
-			foreach (var delimiter in delimiterValues)
-			{
-				// Escape regex special chars to use as regex pattern.
-				var pattern = Regex.Replace(delimiter, @"([.$^{\[(|)*+?\\])", "\\$1");
-				delimiterCounts[delimiter] = Regex.Matches(text, pattern).Count;
-			}
 
-			var maxCount = delimiterCounts.OrderByDescending(c => c.Value).First();
-			if (maxCount.Value > 0)
+			while (text.Length > 0)
 			{
-				delimiter = maxCount.Key;
-				delimiterFirstChar = delimiter[0];
-				configuration.Validate();
+				var index = text.IndexOf(newLine);
+
+				var line = index > -1 ? text.Substring(0, index + newLine.Length) : text;
+
+				var delimiterCounts = new Dictionary<string, int>();
+				foreach (var delimiter in delimiterValues)
+				{
+					// Escape regex special chars to use as regex pattern.
+					var pattern = Regex.Replace(delimiter, @"([.$^{\[(|)*+?\\])", "\\$1");
+					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();
+
+					break;
+				}
+
+				text = index > -1 ? text.Substring(index + newLine.Length) : string.Empty;
 			}
 		}
 

tests/CsvHelper.Tests/Parsing/DetectDelimiterTests.cs

@@ -195,5 +195,47 @@ public void Parse_TextHasRegularCharDelimiter_DetectsDelimiter()
 				Assert.Equal("þ", parser.Delimiter);
 			}
 		}
+
+		[Fact]
+		public void Parse_MultipleLines_DetectsDelimiterFromFirstLineOnly()
+		{
+			var s = new StringBuilder();
+			s.Append("Id;Name\r\n");
+			s.Append("1,2,3,4;5,6,7,8\r\n");
+			var config = new CsvConfiguration(CultureInfo.InvariantCulture)
+			{
+				Delimiter = "`",
+				DetectDelimiter = true,
+				DetectDelimiterValues = new[] { ",", ";" }
+			};
+			using (var reader = new StringReader(s.ToString()))
+			using (var parser = new CsvParser(reader, config))
+			{
+				parser.Read();
+
+				Assert.Equal(";", parser.Delimiter);
+			}
+		}
+
+		[Fact]
+		public void Parse_NoDelimiter_DoesNotDetect()
+		{
+			var s = new StringBuilder();
+			s.Append("Id,Name\r\n");
+			s.Append("1,one\r\n");
+			var config = new CsvConfiguration(CultureInfo.InvariantCulture)
+			{
+				Delimiter = "`",
+				DetectDelimiter = true,
+				DetectDelimiterValues = new[] { ";" }
+			};
+			using (var reader = new StringReader(s.ToString()))
+			using (var parser = new CsvParser(reader, config))
+			{
+				parser.Read();
+
+				Assert.Equal("`", parser.Delimiter);
+			}
+		}
 	}
 }