C# – Setting Margin Properties in code

c++marginwpf

MyControl.Margin.Left = 10;

Error:

Cannot modify the return value of 'System.Windows.FrameworkElement.Margin' because it is not a variable

Best Solution

The problem is that Margin is a property, and its type (Thickness) is a value type. That means when you access the property you're getting a copy of the value back.

Even though you can change the value of the Thickness.Left property for a particular value (grr... mutable value types shouldn't exist), it wouldn't change the margin.

Instead, you'll need to set the Margin property to a new value. For instance (coincidentally the same code as Marc wrote):

Thickness margin = MyControl.Margin;
margin.Left = 10;
MyControl.Margin = margin;

As a note for library design, I would have vastly preferred it if Thickness were immutable, but with methods that returned a new value which was a copy of the original, but with one part replaced. Then you could write:

MyControl.Margin = MyControl.Margin.WithLeft(10);

No worrying about odd behaviour of mutable value types, nice and readable, all one expression...

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In C# 5 and earlier, to give auto implemented properties an initial value, you have to do it in a constructor.

Since C# 6.0, you can specify initial value in-line. The syntax is:

public int X { get; set; } = x; // C# 6 or higher

DefaultValueAttribute is intended to be used by the VS designer (or any other consumer) to specify a default value, not an initial value. (Even if in designed object, initial value is the default value).

At compile time DefaultValueAttribute will not impact the generated IL and it will not be read to initialize the property to that value (see DefaultValue attribute is not working with my Auto Property).

Example of attributes that impact the IL are ThreadStaticAttribute, CallerMemberNameAttribute, ...

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Since Enum Type implements IConvertible interface, a better implementation should be something like this:

public T GetEnumFromString<T>(string value) where T : struct, IConvertible
{
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      throw new ArgumentException("T must be an enumerated type");
   }

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}

This will still permit passing of value types implementing IConvertible. The chances are rare though.

Best Solution

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