PredicateCombinator

A simple fluent api to combine predicates as needed

Background

I've found it necessary several times to author predicates in numerous ways, and it always ends up being extremely difficult to read, write, and work around.

Solution

A simple tree-based predicate combinator which uses a basic fluent api

Usage

Currently, the api only allows for simple fluent creation of either a Predicate<T> or a Func<T,bool> using and/or statements, as well as grouped statements.

Simple Usage

A PredicateBuilder<T> is used to create the predicate. Usage is simple:

// if (s.Contains('a'))
var builder = new PredicateBuilder<string>();
builder.Build()
       .WithPredicate(s => s.Contains('a'))
       .AsFunc();

Of course, more complex predicates can be built:

// if (s.Contains('a') || string.Equals('b'))
builder.Build()
       .WithPredicate(s => s.Contains('a'))
       .Or(s => string.Equals('b')
       .AsFunc();

Groups

Predicates can include groups. These groups are themselves predicate trees which is ultimately added to the main expression tree. See Combinator Logic.

A group requires an Action<IFluentPredicateBuilder<T>> to create the group's expression tree. Again, usage is relatively straightforward, just recursive.

// if ((s.Contains('a') && s.Contains('b')) ||  
builder.Build()
       .WithGroup(b => 
            b.WithPredicate(s => s.Contains('a'))
             .And(s => s.Contains('b')))
       .Or(s => s == "Other Test");

Combinator Logic

Predicates are combined in groups. Groups can either be of And or Or combinators. The first group is always an And group. Whenever a statement is added, if the statements combinator type is the same as the previous statement's, the statement is simply added to the current tree. e.g. if a group is already an And group, and another And statement is added:

builder.Build().WithPredicate(A).And(B).And(C)

results in the equivalent statement A && B && C. Wherever the combinator is switched, a new tree is created with the current tree as the first child node of the new node.

builder.Build()
       .WithPredicate(A)
       .And(B)
       .Or(C)
       .OR(D)
       .And(E)

Results in ((A && B) || C || D) && E