Remove a few more unnecessary StringBuilders

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

@@ -60,24 +60,7 @@ internal void ParseValue(string addresses)
             }
         }
 
-        public override string ToString()
-        {
-            bool first = true;
-            StringBuilder builder = new StringBuilder();
-
-            foreach (MailAddress address in this)
-            {
-                if (!first)
-                {
-                    builder.Append(", ");
-                }
-
-                builder.Append(address.ToString());
-                first = false;
-            }
-
-            return builder.ToString();
-        }
+        public override string ToString() => string.Join(", ", this);
 
         internal string Encode(int charsConsumed, bool allowUnicode)
         {

src/libraries/System.Net.Requests/src/System/Net/FtpControlStream.cs

@@ -806,14 +806,12 @@ private string FormatAddress(IPAddress address, int Port)
         /// </summary>
         private string FormatAddressV6(IPAddress address, int port)
         {
-            StringBuilder sb = new StringBuilder(43); // based on max size of IPv6 address + port + seperators
-            string addressString = address.ToString();
-            sb.Append("|2|");
-            sb.Append(addressString);
-            sb.Append('|');
-            sb.Append(port.ToString(NumberFormatInfo.InvariantInfo));
-            sb.Append('|');
-            return sb.ToString();
+            return
+                "|2|" +
+                address.ToString() +
+                "|" +
+                port.ToString(NumberFormatInfo.InvariantInfo) +
+                "|";
         }
 
         internal long ContentLength
@@ -1121,15 +1119,9 @@ private string GetPortCommandLine(FtpWebRequest request)
         /// </summary>
         private string FormatFtpCommand(string command, string? parameter)
         {
-            StringBuilder stringBuilder = new StringBuilder(command.Length + ((parameter != null) ? parameter.Length : 0) + 3 /*size of ' ' \r\n*/);
-            stringBuilder.Append(command);
-            if (!string.IsNullOrEmpty(parameter))
-            {
-                stringBuilder.Append(' ');
-                stringBuilder.Append(parameter);
-            }
-            stringBuilder.Append("\r\n");
-            return stringBuilder.ToString();
+            return string.IsNullOrEmpty(parameter) ?
+                command + "\r\n" :
+                command + " " + parameter + "\r\n";
         }
 
         /// <summary>

src/libraries/System.Net.Requests/src/System/Net/HttpWebResponse.cs

@@ -96,20 +96,7 @@ public override string ContentType
                 {
                     // In most cases, there is only one media type value as per RFC. But for completeness, we
                     // return all values in cases of overly malformed strings.
-                    var builder = new StringBuilder();
-                    int ndx = 0;
-                    foreach (string value in values)
-                    {
-                        if (ndx > 0)
-                        {
-                            builder.Append(',');
-                        }
-
-                        builder.Append(value);
-                        ndx++;
-                    }
-
-                    return builder.ToString();
+                    return string.Join(',', values);
                 }
                 else
                 {

src/libraries/System.Reflection.MetadataLoadContext/src/System/Reflection/TypeLoading/Events/Ecma/EcmaEvent.cs

@@ -83,12 +83,10 @@ public sealed override string ToString()
             if (disposedString != null)
                 return disposedString;
 
-            StringBuilder sb = new StringBuilder();
-            string typeString = EventDefinition.Type.ToTypeString(TypeContext, Reader);
-            sb.Append(typeString);
-            sb.Append(' ');
-            sb.Append(Name);
-            return sb.ToString();
+            return
+                EventDefinition.Type.ToTypeString(TypeContext, Reader) +
+                " " +
+                Name;
         }
 
         protected sealed override RoMethod? ComputeEventAddMethod() => EventDefinition.GetAccessors().Adder.ToMethodOrNull(GetRoDeclaringType(), ReflectedType);

src/libraries/System.Reflection.MetadataLoadContext/src/System/Reflection/TypeLoading/Fields/Ecma/EcmaField.cs

@@ -79,12 +79,10 @@ public sealed override string ToString()
             if (disposedString != null)
                 return disposedString;
 
-            StringBuilder sb = new StringBuilder();
-            string typeString = FieldDefinition.DecodeSignature(EcmaSignatureTypeProviderForToString.Instance, TypeContext);
-            sb.Append(typeString);
-            sb.Append(' ');
-            sb.Append(Name);
-            return sb.ToString();
+            return
+                FieldDefinition.DecodeSignature(EcmaSignatureTypeProviderForToString.Instance, TypeContext) +
+                " " +
+                Name;
         }
 
         private MetadataReader Reader => _module.Reader;