evaluationengineexpressions.html

---
layout: documentation 
title: EvaluationEngine 
teaser: Decouple business logic from control flow
navigation:
   - name: Overview
     link: evaluationengine.html
   - name: Tutorial
     link: evaluationenginetutorial.html	
   - name: Modules
     link: evaluationenginemodules.html
   - name: Hierarchical Evaluation Engines
     link: evaluationenginehierarchies.html
   - name: Aggregators
     link: evaluationengineaggregators.html
   - name: Expressions
     link: evaluationengineexpressions.html
   - name: Strategies
     link: evaluationenginestrategies.html
   - name: Validation
     link: evaluationenginevalidation.html
   - name: Logging
     link: evaluationenginelogging.html
   - name: Tips and Tricks
     link: evaluationenginetipsandtricks.html
   - name: Specifications
     link: evaluationenginespecifications.html		 
---
<h2>Expressions</h2>
<p>
An expression calculates a part of the result. 
This allows you to add and remove expressions when your requirements change easily. 
</p>

<h3>Inline Expressions</h3>
<p>
Inline expressions are defined directly as lambda expressions:
</p>
<script type="syntaxhighlighter" class="brush: csharp"><![CDATA[
evaluationEngine.Solve<HowCoolIsTheEvaluationEngine, string>()
	.AggregateWithExpressionAggregator(string.Empty, (aggregate, value) => aggregate + " " + value)
	.ByEvaluating((q, p) => "extremely")
	.ByEvaluating((q, p) => "super")
	.ByEvaluating((q, p) => "fantastic");
]]></script>
<p>
Note that you cannot specify a method because a lambda <strong>expression</strong> is required.
</p>
<p>
The expression gets access to the question and the parameter. 
If the question doesn't have a parameter then <code>Missing.Value</code> is passed as the parameter.
</p>

<h3>Expression Classes</h3>
<p>
Normally, your expressions are a bit more complex and writing them directly into the solution definition is not feasible.
Therefore, you can pass an instance of a class implementing <code>IExpression</code> instead of a lambda expression:
</p>
<script type="syntaxhighlighter" class="brush: csharp"><![CDATA[
evaluationEngine.Solve<HowManyVowelsAreInThisText, int, string>()
    .AggregateWithExpressionAggregator(0, (aggregate, value) => aggregate + value)
    .ByEvaluating(q => new CountVowelExpression { Vowel = 'a' })
    .ByEvaluating(q => new CountVowelExpression { Vowel = 'e' })
    .ByEvaluating(q => new CountVowelExpression { Vowel = 'i' })
    .ByEvaluating(q => new CountVowelExpression { Vowel = 'o' })
    .ByEvaluating(q => new CountVowelExpression { Vowel = 'u' })
]]></script>
<p>
You get access to the question so that you can create different expressions based on information from the question.
</p>
<p>
The parameter is passed to the <code>Evaluate</code> method on the expression:
</p>
<script type="syntaxhighlighter" class="brush: csharp"><![CDATA[
public class CountVowelExpression : IExpression<int, string>
{
	public char Vowel { get; set; }

	public int Evaluate(string parameter)
	{
		return parameter.Count(c => c == this.Vowel);
	}

	public string Describe()
	{
		return "counts the number of " + this.Vowel;
	}
}
]]></script>
<p>
Again, for parameter-less questions <code>Missing</code> is used as the type of the paramter and <code>Missing.Value</code> as the value.
</p>
<p>
The text you specify in the <code>Describe</code> method is used when logging is enabled.
</p>