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Caching example, step 1: getting started

[!metalama-project-buttons .]

At first glance, caching appears simple. The aspect generates the cache key, performs a cache lookup, and returns the result if it exists. If it doesn't exist, the method runs, and the return value gets stored in the cache. The following example compares the source code, without caching logic, to the transformed code, with caching logic.

[!metalama-compare Calculator.cs]

Infrastructure Code

The aspect relies on the ICache interface, which has only two methods used by the aspect.

[!metalama-file ICache.cs]

A typical implementation of this interface would use xref:System.Runtime.Caching.MemoryCache.

Aspect Code

The aspect itself is straightforward:

[!metalama-file CacheAttribute.cs]

As usual, the aspect class inherits the abstract class xref:Metalama.Framework.Aspects.OverrideMethodAspect, which, in turn, derives from the xref:System.Attribute?text=System.Attribute class. This makes CacheAttribute a custom attribute. The xref:Metalama.Framework.Aspects.OverrideMethodAspect.OverrideMethod* method acts like a template. Most of its code gets injected into the target method, the one to which we add the [ReportAndSwallowExceptionsAttribute] custom attribute. Inside the xref:Metalama.Framework.Aspects.OverrideMethodAspect.OverrideMethod* implementation, the call to meta.Proceed() has a unique meaning. When the aspect applies to the target, the call to meta.Proceed() stands in for the original implementation, with a few syntactic alterations that capture the return value.

The CacheKeyBuilder class hides the complexity of creating the cache key. It is a compile-time class.

[!metalama-file CacheKeyBuilder.cs]

As its name suggests, the xref:Metalama.Framework.Code.SyntaxBuilders.InterpolatedStringBuilder class helps build interpolated strings. The meta.Target property exposes the context into which the template applies. meta.Target.Type is the current type, meta.Target.Method is the current method, and so on. The GetCachingKey method creates an interpolated string for the current context. Parameters get represented at compile time using the xref:Metalama.Framework.Code.IParameter interface. The xref:Metalama.Framework.Code.IExpression.Value?text=parameter.Value expression returns a dynamic object that represents a run-time expression. In this case, it's the name of the parameter.

Upon receiving the xref:Metalama.Framework.Code.SyntaxBuilders.InterpolatedStringBuilder instance from the CacheKeyBuilder class, the aspect converts it to a run-time interpolated string by calling the xref:Metalama.Framework.Code.SyntaxBuilders.ExpressionBuilderExtensions.ToValue* method. This method returns a dynamic object representing the interpolated string. It can be cast to a string, and we use it as the caching key.

[!div class="see-also"]

  • xref:template-overview
  • xref:Metalama.Framework.Code.SyntaxBuilders.InterpolatedStringBuilder
  • xref:template-compile-time
  • xref:template-dynamic-code