Fix Complex number custom formatting for negative imaginary parts (#46897)

Author: Copilot

docs/fundamentals/runtime-libraries/snippets/System.Numerics/Complex/Overview/csharp/Project.csproj

@@ -2,7 +2,7 @@
 
   <PropertyGroup>
     <OutputType>Exe</OutputType>
-    <TargetFrameworks>net9</TargetFrameworks>
+    <TargetFramework>net9.0</TargetFramework>
   </PropertyGroup>
 
  </Project>

docs/fundamentals/runtime-libraries/snippets/System.Numerics/Complex/Overview/csharp/customfmt1.cs

@@ -33,9 +33,19 @@ public string Format(string format, object arg,
                 fmtString = "N" + precision.ToString();
             }
             if (format.Substring(0, 1).Equals("I", StringComparison.OrdinalIgnoreCase))
-                return c1.Real.ToString(fmtString) + " + " + c1.Imaginary.ToString(fmtString) + "i";
+            {
+                // Determine the sign to display.
+                char sign = c1.Imaginary < 0 ? '-' : '+';
+                // Display the determined sign and the absolute value of the imaginary part.
+                return c1.Real.ToString(fmtString) + " " + sign + " " + Math.Abs(c1.Imaginary).ToString(fmtString) + "i";
+            }
             else if (format.Substring(0, 1).Equals("J", StringComparison.OrdinalIgnoreCase))
-                return c1.Real.ToString(fmtString) + " + " + c1.Imaginary.ToString(fmtString) + "j";
+            {
+                // Determine the sign to display.
+                char sign = c1.Imaginary < 0 ? '-' : '+';
+                // Display the determined sign and the absolute value of the imaginary part.
+                return c1.Real.ToString(fmtString) + " " + sign + " " + Math.Abs(c1.Imaginary).ToString(fmtString) + "j";
+            }
             else
                 return c1.ToString(format, provider);
         }

docs/fundamentals/runtime-libraries/snippets/System.Numerics/Complex/Overview/vb/Project.vbproj

@@ -2,7 +2,7 @@
 
   <PropertyGroup>
     <OutputType>Exe</OutputType>
-    <TargetFrameworks>net9</TargetFrameworks>
+    <TargetFramework>net9.0</TargetFramework>
   </PropertyGroup>
 
  </Project>

docs/fundamentals/runtime-libraries/snippets/System.Numerics/Complex/Overview/vb/customfmt1.vb

@@ -32,10 +32,13 @@ Public Class ComplexFormatter
                 End Try
                 fmtString = "N" + precision.ToString()
             End If
+            ' Determine the sign to display.
+            Dim sign As Char = If(c1.Imaginary < 0.0, "-"c, "+"c)
+            ' Display the determined sign and the absolute value of the imaginary part.
             If fmt.Substring(0, 1).Equals("I", StringComparison.OrdinalIgnoreCase) Then
-                Return c1.Real.ToString(fmtString) + " + " + c1.Imaginary.ToString(fmtString) + "i"
+                Return c1.Real.ToString(fmtString) + " " + sign + " " + Math.Abs(c1.Imaginary).ToString(fmtString) + "i"
             ElseIf fmt.Substring(0, 1).Equals("J", StringComparison.OrdinalIgnoreCase) Then
-                Return c1.Real.ToString(fmtString) + " + " + c1.Imaginary.ToString(fmtString) + "j"
+                Return c1.Real.ToString(fmtString) + " " + sign + " " + Math.Abs(c1.Imaginary).ToString(fmtString) + "j"
             Else
                 Return c1.ToString(fmt, provider)
             End If