overriding-properties.md

uid	level	summary
simple-override-property	200	The document provides a guide on how to override fields and properties using the Metalama.Framework package in C#, including examples of trimming strings and normalizing values to uppercase.

Getting started: overriding fields and properties

In xref:simple-override-method, you learned how to change the implementation of a method with an aspect. You can apply the same technique to fields and properties by extending xref:Metalama.Framework.Aspects.OverrideFieldOrPropertyAspect.

Overriding a field or property

Follow these steps to create an aspect capable of overriding a field or property:

1. Add the Metalama.Framework package to your project.

2. Create a new class derived from the xref:Metalama.Framework.Aspects.OverrideFieldOrPropertyAspect abstract class. This class will serve as a custom attribute, so it's recommended to name it with the Attribute suffix.

3. Implement the xref:Metalama.Framework.Aspects.OverrideFieldOrPropertyAspect.OverrideProperty property in plain C#. To call the original implementation, use xref:Metalama.Framework.Aspects.meta.Proceed?text=meta.Proceed.

4. Since the aspect is a custom attribute, you can transform a field or property by simply adding the aspect custom attribute to the field or property.

Warning

When applying an aspect to a field, Metalama will automatically transform the field into a property. If the field is used by reference using ref, out, and in keywords, it will result in a compile-time error.

Trick: an empty OverrideFieldOrPropertyAspect aspect

The following example demonstrates an empty implementation of xref:Metalama.Framework.Aspects.OverrideFieldOrPropertyAspect applied to a property and a field.

[!metalama-test ~/code/Metalama.Documentation.SampleCode.AspectFramework/EmptyOverrideFieldOrProperty.cs name="Empty OverrideFieldOrProperty"]

This aspect doesn't perform any specific function, but as you can see, it transforms the field into a property.

Getting or setting the underlying property

If you've only worked with methods so far, you're likely familiar with using the meta.Proceed() method in your template. This method also works in a property template: when called from the getter, it returns the field or property value; when called from the setter, it sets the field or property to the value of the value parameter.

If you need to get the property value from the setter, or if you need to set the property value to something other than the value parameter, you can do so by getting or setting the xref:Metalama.Framework.Code.IExpression.Value?text=meta.Target.FieldOrProperty.Value property.

Example: trimming strings

In this aspect, you will see how you can trim whitespace before and after string values before they are assigned to the field or property.

[!metalama-test ~/code/Metalama.Documentation.SampleCode.EnhanceProperties/Trimmed.cs name="Trimming string fields and properties"]

The aspect doesn't need to modify the getter, so it only calls meta.Proceed(), and Metalama replaces this call with the original implementation of the property. We could have written get => meta.Target.PropertyOrField.Value instead, achieving the same effect.

The setter is modified to call the Trim method on the input value. The most concise and simple code is set => meta.Target.PropertyOrField.Value = value?.Trim. Alternatively, we could have written the following code:

set
{
    value = value?.Trim();
    meta.Proceed();
}

Example: turning the value to upper case

The following example is similar to the previous one, but instead of trimming a string, we normalize it to upper case.

We apply the aspect to a class representing a shipment between two airports.

[!metalama-test ~/code/Metalama.Documentation.SampleCode.EnhanceProperties/UpperCase.cs name="Changing case to Upper case"]

Note that in this example, From is a public field and To is a public property. They are deliberately kept that way to demonstrate that the aspect works on both because xref:Metalama.Framework.Code.IFieldOrProperty is used in the aspect. If you want the aspect to apply only to properties and not to fields, you should use xref:Metalama.Framework.Code.IProperty.

Going deeper

If you want to go deeper into field/property overrides, consider exploring the following articles:

• In this article, you learned how to use meta.Proceed and meta.Target.Method.Name in your templates. You can write much more complex and powerful templates, even performing compile-time if and foreach blocks. To see how, you can jump directly to xref:templates.

• To learn how to override several fields and properties from a single type-level aspect, jump to xref:overriding-methods.

[!div class="see-also"] xref:video-other-aspect-types