The statement under "Remarks" is not correct: If the values for Top, Bottom, Left, or Rights were set separately, then All returns -1, even if all 4 properties have the same value:
var p = new Padding( 5 );
// Here, All returns 5
p.Left = 5;
// Now, All returns -1, although all 4 properties still have a value of 5
p.All = 5;
// Now, All returns 5 again

see also: https://referencesource.microsoft.com/#System.Windows.Forms/winforms/Managed/System/WinForms/Padding.cs,6c9dc153b2c496ae

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• ID: 0c67d24a-2485-92bb-9e66-87104cf3eb61
• Version Independent ID: 9e56ed04-9020-f76c-a783-26ac11dfbee6
• Content: Padding.All Property (System.Windows.Forms)
• Content Source: xml/System.Windows.Forms/Padding.xml
• Service: unspecified
• GitHub Login: @dotnet-bot
• Microsoft Alias: dotnetcontent