diff --git a/csharp/src/Google.Protobuf/Collections/MapField.cs b/csharp/src/Google.Protobuf/Collections/MapField.cs
index 722cc92322ef..e9a106e1068e 100644
--- a/csharp/src/Google.Protobuf/Collections/MapField.cs
+++ b/csharp/src/Google.Protobuf/Collections/MapField.cs
@@ -590,7 +590,7 @@ object IDictionary.this[object key]
         IEnumerable<TValue> IReadOnlyDictionary<TKey, TValue>.Values => Values;
         #endregion
 
-        private class DictionaryEnumerator : IDictionaryEnumerator
+        private sealed class DictionaryEnumerator : IDictionaryEnumerator
         {
             private readonly IEnumerator<KeyValuePair<TKey, TValue>> enumerator;
 
diff --git a/csharp/src/Google.Protobuf/JsonFormatter.cs b/csharp/src/Google.Protobuf/JsonFormatter.cs
index fbcc83970720..b420ae615193 100644
--- a/csharp/src/Google.Protobuf/JsonFormatter.cs
+++ b/csharp/src/Google.Protobuf/JsonFormatter.cs
@@ -85,7 +85,7 @@ public sealed class JsonFormatter {
 
     static JsonFormatter() {
       for (int i = 0; i < CommonRepresentations.Length; i++) {
-        if (CommonRepresentations[i] == "") {
+        if (CommonRepresentations[i].Length == 0) {
           CommonRepresentations[i] = ((char)i).ToString();
         }
       }
@@ -291,7 +291,7 @@ private static bool IsDefaultValue(FieldDescriptor descriptor, object value) {
       return descriptor.FieldType switch {
         FieldType.Bool => (bool)value == false,
         FieldType.Bytes => (ByteString)value == ByteString.Empty,
-        FieldType.String => (string)value == "",
+        FieldType.String => ((string)value).Length == 0,
         FieldType.Double => (double)value == 0.0,
         FieldType.SInt32 or FieldType.Int32 or FieldType.SFixed32 or FieldType.Enum =>
             (int)value == 0,
@@ -497,7 +497,7 @@ private void WriteAny(TextWriter writer, IMessage value, int indentationLevel) {
       WriteBracketClose(writer, ObjectCloseBracket, true, indentationLevel);
     }
 
-    private void WriteDiagnosticOnlyAny(TextWriter writer, IMessage value) {
+    private static void WriteDiagnosticOnlyAny(TextWriter writer, IMessage value) {
       string typeUrl =
           (string)value.Descriptor.Fields[Any.TypeUrlFieldNumber].Accessor.GetValue(value);
       ByteString data =
diff --git a/csharp/src/Google.Protobuf/JsonParser.cs b/csharp/src/Google.Protobuf/JsonParser.cs
index fcf859daba09..095875cab326 100644
--- a/csharp/src/Google.Protobuf/JsonParser.cs
+++ b/csharp/src/Google.Protobuf/JsonParser.cs
@@ -830,7 +830,7 @@ private static void MergeTimestamp(IMessage message, JsonToken token)
                 // TODO: It would be nice not to have to create all these objects... easy to optimize later though.
                 Timestamp timestamp = Timestamp.FromDateTime(parsed);
                 int nanosToAdd = 0;
-                if (subseconds != "")
+                if (subseconds.Length != 0)
                 {
                     // This should always work, as we've got 1-9 digits.
                     int parsedFraction = int.Parse(subseconds.Substring(1), CultureInfo.InvariantCulture);
@@ -906,7 +906,7 @@ private static void MergeDuration(IMessage message, JsonToken token)
             {
                 long seconds = long.Parse(secondsText, CultureInfo.InvariantCulture) * multiplier;
                 int nanos = 0;
-                if (subseconds != "")
+                if (subseconds.Length != 0)
                 {
                     // This should always work, as we've got 1-9 digits.
                     int parsedFraction = int.Parse(subseconds.Substring(1));
diff --git a/csharp/src/Google.Protobuf/JsonTokenizer.cs b/csharp/src/Google.Protobuf/JsonTokenizer.cs
index f89e36adf4df..d12d427d4f9e 100644
--- a/csharp/src/Google.Protobuf/JsonTokenizer.cs
+++ b/csharp/src/Google.Protobuf/JsonTokenizer.cs
@@ -413,7 +413,7 @@ private double ReadNumber(char initialCharacter)
                 var builder = StringBuilderCache.Acquire();
                 if (initialCharacter == '-')
                 {
-                    builder.Append("-");
+                    builder.Append('-');
                 }
                 else
                 {
diff --git a/csharp/src/Google.Protobuf/Reflection/DescriptorPool.cs b/csharp/src/Google.Protobuf/Reflection/DescriptorPool.cs
index 7289e44a0f56..1fe9465751e3 100644
--- a/csharp/src/Google.Protobuf/Reflection/DescriptorPool.cs
+++ b/csharp/src/Google.Protobuf/Reflection/DescriptorPool.cs
@@ -279,7 +279,7 @@ internal IDescriptor LookupSymbol(string name, IDescriptor relativeTo)
                 {
                     // Chop off the last component of the scope.
 
-                    int dotpos = scopeToTry.ToString().LastIndexOf(".");
+                    int dotpos = scopeToTry.ToString().LastIndexOf('.');
                     if (dotpos == -1)
                     {
                         result = FindSymbol<IDescriptor>(name);
diff --git a/csharp/src/Google.Protobuf/Reflection/FieldDescriptor.cs b/csharp/src/Google.Protobuf/Reflection/FieldDescriptor.cs
index 0654a25f5b28..648b37ad30aa 100644
--- a/csharp/src/Google.Protobuf/Reflection/FieldDescriptor.cs
+++ b/csharp/src/Google.Protobuf/Reflection/FieldDescriptor.cs
@@ -120,7 +120,7 @@ internal FieldDescriptor(FieldDescriptorProto proto, FileDescriptor file,
             // a MapField, but that feels a tad nasty.
             PropertyName = propertyName;
             Extension = extension;
-            JsonName =  Proto.JsonName == "" ? JsonFormatter.ToJsonName(Proto.Name) : Proto.JsonName;
+            JsonName =  Proto.JsonName.Length == 0 ? JsonFormatter.ToJsonName(Proto.Name) : Proto.JsonName;
         }
 
         /// <summary>
diff --git a/csharp/src/Google.Protobuf/Reflection/FileDescriptor.cs b/csharp/src/Google.Protobuf/Reflection/FileDescriptor.cs
index c7b307b1c85d..773d5c2a8718 100644
--- a/csharp/src/Google.Protobuf/Reflection/FileDescriptor.cs
+++ b/csharp/src/Google.Protobuf/Reflection/FileDescriptor.cs
@@ -63,7 +63,7 @@ static FileDescriptor()
 
         private readonly Lazy<Dictionary<IDescriptor, DescriptorDeclaration>> declarations;
 
-        private FileDescriptor(ByteString descriptorData, FileDescriptorProto proto, IEnumerable<FileDescriptor> dependencies, DescriptorPool pool, bool allowUnknownDependencies, GeneratedClrTypeInfo generatedCodeInfo)
+        private FileDescriptor(ByteString descriptorData, FileDescriptorProto proto, IList<FileDescriptor> dependencies, DescriptorPool pool, bool allowUnknownDependencies, GeneratedClrTypeInfo generatedCodeInfo)
         {
             SerializedData = descriptorData;
             DescriptorPool = pool;
@@ -128,7 +128,7 @@ private IDescriptor FindDescriptorForPath(IList<int> path)
             return current;
         }
 
-        private DescriptorBase GetDescriptorFromList(IReadOnlyList<DescriptorBase> list, int index)
+        private static DescriptorBase GetDescriptorFromList(IReadOnlyList<DescriptorBase> list, int index)
         {
             // This is fine: it may be a newer version of protobuf than we understand, with a new descriptor
             // field.
@@ -419,7 +419,7 @@ public static FileDescriptor FromGeneratedCode(
         {
             ExtensionRegistry registry = new ExtensionRegistry();
             registry.AddRange(GetAllExtensions(dependencies, generatedCodeInfo));
-    
+
             FileDescriptorProto proto;
             try
             {