LINQ-Like Criteria Syntax

• Feb 07, 2022
• 3 minutes to read

The CriteriaOperator class implements the CriteriaOperator.FromLambda method. Use this method to create type-safe criteria expressions from lambda expressions.

C#

// Regular criteria syntax
CriteriaOperator opParsed = CriteriaOperator.Parse("[Name] = 'Alex'");

// LINQ-style criteria syntax typed
CriteriaOperator opLambda = CriteriaOperator.FromLambda<Customer, int>(customer => customer.Orders.Count())
// LINQ-style criteria syntax bool only
CriteriaOperator opLambda = CriteriaOperator.FromLambda<Customer>(customer => customer.Name == "Alex")

Overview

CriteriaOperator.FromLambda allows you to build criteria in an intuitive manner:

C#

CriteriaOperator.FromLambda<Customer>(customer => customer.Age > 10);

CriteriaOperator.FromLambda<Customer>(customer => customer.Name == "Alex");

CriteriaOperator.FromLambda<Customer>(customer => customer.IsLongterm);

CriteriaOperator.FromLambda supports LINQ extension methods:

C#

CriteriaOperator.FromLambda<Supplier>(supplier => supplier.Products.Any());

CriteriaOperator.FromLambda<Supplier>(supplier => supplier.Products.All());

CriteriaOperator.FromLambda<Supplier>(supplier => supplier.Products.Average(product => product.Price) > 100 );

Supported Features

Built-in and Custom Functions

CriteriaOperator.FromLambda supports built-in functions of the String, Math, Convert, and DateTime types.

C#

CriteriaOperator.FromLambda<Shape>(Shape  => Shape.Angle < Math.Acos(value));

CriteriaOperator.FromLambda<Customer>(customer  => customer.Name.Trim() = 'Alex');

You can implement custom functions and use them to build criteria operators. See How to: Implement a Custom Criteria Language Function Operator for more information.

C#

static class MyCustomAddFunction : ICustomFunctionOperator {
    string Name {
        get { return "MyCustomAdd"; } 
    }
    // Name must match the Name string
    public static int MyCustomAdd(int a, int b){ 
    } 
    // Interface implementation
}

Built-in and Custom Aggregate Functions

CriteriaOperator.FromLambda supports both custom and standard (Min, Max, Sum, Avg, Single, Exists, and Count) aggregate functions.

C#

CriteriaOperator.FromLambda<Supplier>(supplier  => supplier.Products.CountDistinct < 10);

To use custom aggregates, implement an extension method to the IEnumerable interface in addition an ICustomAggregate interface implementation.

C#

public class CountDistinctCustomAggregate : ICustomAggregate {
    // implementation
}
static class CountDistinctCustomAggregateExtension : IEnumerable {
    int CountDistinct(this IEnumerable instance) {
        // implementation
    }
}

FromLambdaFunctions Helper

Use the FromLambdaFunction helper class to create advanced criteria. This class exposes functions that can be used within the CriteriaOperator.FromLambda method.

Not Type-Safe Custom Functions Including Non-Deterministic

C#

// Creates the following expression: FirstOrderDate([Order])
CriteriaOperator.FromLambda<Customer, DateTime>(customer => 
    FromLambdaFunctions.CustomFunction<DateTime>("FirstOrderDate", customer.Order));
// Creates the following expression: CustomNonDeterministic('FirstOrderDate', [Order])
CriteriaOperator.FromLambda<Customer, DateTime>(customer => 
    FromLambdaFunctions.CustomFunctionNonDeterministic<DateTime>("FirstOrderDate", customer.Order));

Tip

A component that consumes the resulting FunctionOperator may assume that a function always returns the same result for the same input values. Use the CustomNonDeterministic method if a result of a custom function is not predictable.

Top Level Aggregate

All built-in Aggregates (like Count, Exists, and others) are supported:

C#

CriteriaOperator.FromLambda<Customer, int>(customer => FromLambdaFunctions.TopLevelAggregate<Customer>().Count());

CriteriaOperator.FromLambda<Customer, int>(customer => FromLambdaFunctions.TopLevelAggregate<Customer>().Max(im => im.IntProperty));

Free Join

C#

CriteriaOperator.FromLambda<Customer, bool>(customer => FromLambdaFunctions.FreeJoin<Order>(om => (customer.IntProperty == customer.RefProperty) && (om.IntProperty > 10)).Any());

How It Works

CriteriaOperator.FromLambda is designed to build criteria expressions in a LINQ notation. Do not use CriteriaOperator.FromLambda to convert LINQ expressions to Criteria expressions.

CriteriaOperator.FromLambda supports only filtering expressions. Extensions like OrderBy, GroupBy are not supported.

C#

// This code throws an exception
CriteriaOperator fromLambda = CriteriaOperator.FromLambda<Customer, IEnumerable>(customer  => customer.Orders.OrderBy(tm => tm.Name));