Checking strings against length seem to have better perf

src/libraries/Common/src/Interop/Linux/cgroups/Interop.cgroups.cs

@@ -348,7 +348,7 @@ internal static bool TryFindCGroupPathForSubsystem(CGroupVersion cgroupVersion,
                                 // cgroup v2: Find the first entry that matches the cgroup v2 hierarchy:
                                 //     0::$PATH
 
-                                if ((lineParts[0] == "0") && (lineParts[1] == string.Empty))
+                                if ((lineParts[0] == "0") && (lineParts[1].Length == 0))
                                 {
                                     path = lineParts[2];
                                     return true;

src/libraries/Microsoft.Extensions.Logging.EventSource/src/LoggingEventSource.cs

@@ -329,7 +329,7 @@ private void FireChangeToken()
         [NonEvent]
         private static LoggerFilterRule[] ParseFilterSpec(string filterSpec, LogLevel defaultLevel)
         {
-            if (filterSpec == string.Empty)
+            if (filterSpec.Length == 0)
             {
                 return new[] { new LoggerFilterRule(typeof(EventSourceLoggerProvider).FullName, null, defaultLevel, null) };
             }

src/libraries/System.Diagnostics.PerformanceCounter/src/System/Diagnostics/PerformanceCounter.cs

@@ -470,9 +470,9 @@ private void InitializeImpl()
                     string currentCategoryName = _categoryName;
                     string currentMachineName = _machineName;
 
-                    if (currentCategoryName == string.Empty)
+                    if (currentCategoryName.Length == 0)
                         throw new InvalidOperationException(SR.CategoryNameMissing);
-                    if (_counterName == string.Empty)
+                    if (_counterName.Length == 0)
                         throw new InvalidOperationException(SR.CounterNameMissing);
 
                     if (ReadOnly)

src/libraries/System.Diagnostics.PerformanceCounter/src/System/Diagnostics/PerformanceCounterLib.cs

@@ -455,7 +455,7 @@ private static void CreateIniFile(string categoryName, string categoryHelp, Coun
                         iniWriter.Write(languageId);
                         iniWriter.Write(HelpSufix);
                         iniWriter.Write("=");
-                        if (categoryHelp == null || categoryHelp == string.Empty)
+                        if (string.IsNullOrEmpty(categoryHelp))
                             iniWriter.WriteLine(SR.HelpNotAvailable);
                         else
                             iniWriter.WriteLine(categoryHelp);

src/libraries/System.Diagnostics.Process/src/System/Diagnostics/Process.Windows.cs

@@ -536,7 +536,7 @@ private unsafe bool StartWithCreateProcess(ProcessStartInfo startInfo)
                         environmentBlock = GetEnvironmentVariablesBlock(startInfo._environmentVariables!);
                     }
                     string workingDirectory = startInfo.WorkingDirectory;
-                    if (workingDirectory == string.Empty)
+                    if (workingDirectory.Length == 0)
                         workingDirectory = Directory.GetCurrentDirectory();
 
                     bool retVal;

src/libraries/System.Drawing.Common/src/System/Drawing/BitmapSelector.cs

@@ -64,7 +64,7 @@ internal static string AppendSuffix(string filePath)
         /// </summary>
         public static string GetFileName(string originalPath)
         {
-            if (Suffix == string.Empty)
+            if (string.IsNullOrEmpty(Suffix))
                 return originalPath;
 
             string newPath = AppendSuffix(originalPath);

src/libraries/System.IO.IsolatedStorage/src/System/IO/IsolatedStorage/IsolatedStorageFile.cs

@@ -262,7 +262,7 @@ public DateTimeOffset GetCreationTime(string path)
             if (path == null)
                 throw new ArgumentNullException(nameof(path));
 
-            if (path == string.Empty)
+            if (path.Length == 0)
             {
                 throw new ArgumentException(SR.Argument_EmptyPath, nameof(path));
             }
@@ -284,7 +284,7 @@ public DateTimeOffset GetLastAccessTime(string path)
             if (path == null)
                 throw new ArgumentNullException(nameof(path));
 
-            if (path == string.Empty)
+            if (path.Length == 0)
             {
                 throw new ArgumentException(SR.Argument_EmptyPath, nameof(path));
             }
@@ -306,7 +306,7 @@ public DateTimeOffset GetLastWriteTime(string path)
             if (path == null)
                 throw new ArgumentNullException(nameof(path));
 
-            if (path == string.Empty)
+            if (path.Length == 0)
             {
                 throw new ArgumentException(SR.Argument_EmptyPath, nameof(path));
             }
@@ -331,12 +331,12 @@ public void CopyFile(string sourceFileName, string destinationFileName)
             if (destinationFileName == null)
                 throw new ArgumentNullException(nameof(destinationFileName));
 
-            if (sourceFileName == string.Empty)
+            if (sourceFileName.Length == 0)
             {
                 throw new ArgumentException(SR.Argument_EmptyPath, nameof(sourceFileName));
             }
 
-            if (destinationFileName == string.Empty)
+            if (destinationFileName.Length == 0)
             {
                 throw new ArgumentException(SR.Argument_EmptyPath, nameof(destinationFileName));
             }
@@ -352,12 +352,12 @@ public void CopyFile(string sourceFileName, string destinationFileName, bool ove
             if (destinationFileName == null)
                 throw new ArgumentNullException(nameof(destinationFileName));
 
-            if (sourceFileName == string.Empty)
+            if (sourceFileName.Length == 0)
             {
                 throw new ArgumentException(SR.Argument_EmptyPath, nameof(sourceFileName));
             }
 
-            if (destinationFileName == string.Empty)
+            if (destinationFileName.Length == 0)
             {
                 throw new ArgumentException(SR.Argument_EmptyPath, nameof(destinationFileName));
             }
@@ -393,12 +393,12 @@ public void MoveFile(string sourceFileName, string destinationFileName)
             if (destinationFileName == null)
                 throw new ArgumentNullException(nameof(destinationFileName));
 
-            if (sourceFileName == string.Empty)
+            if (sourceFileName.Length == 0)
             {
                 throw new ArgumentException(SR.Argument_EmptyPath, nameof(sourceFileName));
             }
 
-            if (destinationFileName == string.Empty)
+            if (destinationFileName.Length == 0)
             {
                 throw new ArgumentException(SR.Argument_EmptyPath, nameof(destinationFileName));
             }
@@ -434,12 +434,12 @@ public void MoveDirectory(string sourceDirectoryName, string destinationDirector
             if (destinationDirectoryName == null)
                 throw new ArgumentNullException(nameof(destinationDirectoryName));
 
-            if (sourceDirectoryName == string.Empty)
+            if (sourceDirectoryName.Length == 0)
             {
                 throw new ArgumentException(SR.Argument_EmptyPath, nameof(sourceDirectoryName));
             }
 
-            if (destinationDirectoryName == string.Empty)
+            if (destinationDirectoryName.Length == 0)
             {
                 throw new ArgumentException(SR.Argument_EmptyPath, nameof(destinationDirectoryName));
             }

src/libraries/System.IO.Packaging/src/System/IO/Packaging/InternalRelationshipCollection.cs

@@ -161,7 +161,7 @@ internal void Flush()
         internal static void ThrowIfInvalidRelationshipType(string relationshipType)
         {
             // Look for empty string or string with just spaces
-            if (relationshipType.Trim() == string.Empty)
+            if (relationshipType.Trim().Length == 0)
                 throw new ArgumentException(SR.InvalidRelationshipType);
         }
 

src/libraries/System.IO.Packaging/src/System/IO/Packaging/PackUriHelper.PackUriScheme.cs

@@ -79,7 +79,7 @@ public static Uri Create(Uri packageUri, Uri partUri, string fragment)
 
             if (fragment != null)
             {
-                if (fragment == string.Empty || fragment[0] != '#')
+                if (fragment.Length == 0 || fragment[0] != '#')
                     throw new ArgumentException(SR.Format(SR.FragmentMustStartWithHash, nameof(fragment)));
             }
 
@@ -323,7 +323,7 @@ private static PackUriHelper.ValidatedPartUri GetPartUriComponent(Uri packUri)
 
             string partName = GetStringForPartUriFromAnyUri(packUri);
 
-            if (partName == string.Empty)
+            if (partName.Length == 0)
                 return null;
             else
                 return ValidatePartUri(new Uri(partName, UriKind.Relative));

src/libraries/System.IO.Packaging/src/System/IO/Packaging/PackUriHelper.cs

@@ -49,7 +49,7 @@ public static Uri CreatePartUri(Uri partUri)
 
             string partName = GetStringForPartUriFromAnyUri(resolvedUri);
 
-            if (partName == string.Empty)
+            if (partName.Length == 0)
                 throw new ArgumentException(SR.PartUriIsEmpty);
 
             ThrowIfPartNameEndsWithSlash(partName);
@@ -399,7 +399,7 @@ private static Exception GetExceptionIfPartUriInvalid(Uri partUri, out string pa
 
             //We need to make sure that the URI passed to us is not just "/"
             //"/" is a valid relative uri, but is not a valid partname
-            if (partName == string.Empty)
+            if (partName.Length == 0)
                 return new ArgumentException(SR.PartUriIsEmpty);
 
             if (partName[0] != ForwardSlashChar)
@@ -804,7 +804,7 @@ private bool IsRelationshipUri()
                 // String.Split, will always return an empty string as the
                 // first member in the array as the string starts with a "/"
 
-                Debug.Assert(segments.Length > 0 && segments[0] == string.Empty);
+                Debug.Assert(segments.Length > 0 && segments[0].Length == 0);
 
                 //If the extension was not equal to .rels, we would have exited early.
                 Debug.Assert(string.CompareOrdinal((Path.GetExtension(segments[segments.Length - 1])), RelationshipPartUpperCaseExtension) == 0);

src/libraries/System.IO.Packaging/src/System/IO/Packaging/ZipPackage.cs

@@ -964,7 +964,7 @@ private void ValidateXmlAttribute(string attributeName, string attributeValue, s
                 ThrowIfXmlAttributeMissing(attributeName, attributeValue, tagName, reader);
 
                 //Checking for empty attribute
-                if (attributeValue == string.Empty)
+                if (attributeValue.Length == 0)
                     throw new XmlException(SR.Format(SR.RequiredAttributeEmpty, tagName, attributeName), null, ((IXmlLineInfo)reader).LineNumber, ((IXmlLineInfo)reader).LinePosition);
             }
 

src/libraries/System.Net.Http/src/System/Net/Http/Headers/HttpRequestHeaders.cs

@@ -98,7 +98,7 @@ public string? From
             set
             {
                 // Null and empty string are equivalent. In this case it means, remove the From header value (if any).
-                if (value == string.Empty)
+                if (value?.Length == 0)
                 {
                     value = null;
                 }
@@ -117,7 +117,7 @@ public string? Host
             set
             {
                 // Null and empty string are equivalent. In this case it means, remove the Host header value (if any).
-                if (value == string.Empty)
+                if (value?.Length == 0)
                 {
                     value = null;
                 }

src/libraries/System.Net.Mail/src/System/Net/Mail/Attachment.cs

@@ -75,7 +75,7 @@ internal void SetContentFromFile(string fileName, ContentType? contentType)
                 throw new ArgumentNullException(nameof(fileName));
             }
 
-            if (fileName == string.Empty)
+            if (fileName.Length == 0)
             {
                 throw new ArgumentException(SR.Format(SR.net_emptystringcall, nameof(fileName)), nameof(fileName));
             }
@@ -91,7 +91,7 @@ internal void SetContentFromFile(string fileName, string? mediaType)
                 throw new ArgumentNullException(nameof(fileName));
             }
 
-            if (fileName == string.Empty)
+            if (fileName.Length == 0)
             {
                 throw new ArgumentException(SR.Format(SR.net_emptystringcall, nameof(fileName)), nameof(fileName));
             }
@@ -154,7 +154,7 @@ internal void SetContentFromString(string content, Encoding? encoding, string? m
                 _part.Stream.Close();
             }
 
-            if (mediaType == null || mediaType == string.Empty)
+            if (string.IsNullOrEmpty(mediaType))
             {
                 mediaType = MediaTypeNames.Text.Plain;
             }
@@ -340,7 +340,7 @@ public Attachment(string fileName, string? mediaType) :
         public Attachment(string fileName, ContentType contentType) :
             base(fileName, contentType)
         {
-            if (contentType.Name == null || contentType.Name == string.Empty)
+            if (string.IsNullOrEmpty(contentType.Name))
             {
                 Name = Path.GetFileName(fileName);
             }

src/libraries/System.Net.Mail/src/System/Net/Mail/MailAddress.cs

@@ -124,7 +124,7 @@ private static bool TryParse(string address, string? displayName, Encoding? disp
             {
                 throw new ArgumentNullException(nameof(address));
             }
-            if (address == string.Empty)
+            if (address.Length == 0)
             {
                 throw new ArgumentException(SR.Format(SR.net_emptystringcall, nameof(address)), nameof(address));
             }

src/libraries/System.Net.Mail/src/System/Net/Mail/MailAddressCollection.cs

@@ -23,7 +23,7 @@ public void Add(string addresses)
             {
                 throw new ArgumentNullException(nameof(addresses));
             }
-            if (addresses == string.Empty)
+            if (addresses.Length == 0)
             {
                 throw new ArgumentException(SR.Format(SR.net_emptystringcall, nameof(addresses)), nameof(addresses));
             }

src/libraries/System.Net.Mail/src/System/Net/Mail/MailMessage.cs

@@ -44,10 +44,10 @@ public MailMessage(string from, string to)
             if (to == null)
                 throw new ArgumentNullException(nameof(to));
 
-            if (from == string.Empty)
+            if (from.Length == 0)
                 throw new ArgumentException(SR.Format(SR.net_emptystringcall, nameof(from)), nameof(from));
 
-            if (to == string.Empty)
+            if (to.Length == 0)
                 throw new ArgumentException(SR.Format(SR.net_emptystringcall, nameof(to)), nameof(to));
 
             _message = new Message(from, to);

src/libraries/System.Net.Mail/src/System/Net/Mail/MailPriority.cs

@@ -51,10 +51,10 @@ internal Message(string from, string to) : this()
             if (to == null)
                 throw new ArgumentNullException(nameof(to));
 
-            if (from == string.Empty)
+            if (from.Length == 0)
                 throw new ArgumentException(SR.Format(SR.net_emptystringcall, nameof(from)), nameof(from));
 
-            if (to == string.Empty)
+            if (to.Length == 0)
                 throw new ArgumentException(SR.Format(SR.net_emptystringcall, nameof(to)), nameof(to));
 
             _from = new MailAddress(from);

src/libraries/System.Net.Mail/src/System/Net/Mail/SmtpClient.cs

@@ -187,7 +187,7 @@ public string? Host
                     throw new ArgumentNullException(nameof(value));
                 }
 
-                if (value == string.Empty)
+                if (value?.Length == 0)
                 {
                     throw new ArgumentException(SR.net_emptystringset, nameof(value));
                 }

src/libraries/System.Net.Mail/src/System/Net/Mime/ContentDisposition.cs

@@ -83,7 +83,7 @@ public string DispositionType
                 {
                     throw new ArgumentNullException(nameof(value));
                 }
-                if (value == string.Empty)
+                if (value.Length == 0)
                 {
                     throw new ArgumentException(SR.net_emptystringset, nameof(value));
                 }

src/libraries/System.Net.Mail/src/System/Net/Mime/ContentType.cs

@@ -50,7 +50,7 @@ public ContentType(string contentType)
             {
                 throw new ArgumentNullException(nameof(contentType));
             }
-            if (contentType == string.Empty)
+            if (contentType.Length == 0)
             {
                 throw new ArgumentException(SR.Format(SR.net_emptystringcall, nameof(contentType)), nameof(contentType));
             }
@@ -65,7 +65,7 @@ public string? Boundary
             get { return Parameters["boundary"]; }
             set
             {
-                if (value == null || value == string.Empty)
+                if (string.IsNullOrEmpty(value))
                 {
                     Parameters.Remove("boundary");
                 }
@@ -81,7 +81,7 @@ public string? CharSet
             get { return Parameters["charset"]; }
             set
             {
-                if (value == null || value == string.Empty)
+                if (string.IsNullOrEmpty(value))
                 {
                     Parameters.Remove("charset");
                 }
@@ -105,7 +105,7 @@ public string MediaType
                     throw new ArgumentNullException(nameof(value));
                 }
 
-                if (value == string.Empty)
+                if (value.Length == 0)
                 {
                     throw new ArgumentException(SR.net_emptystringset, nameof(value));
                 }