LINQ-Like Criteria Syntax

• Mar 28, 2024
• 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, DateTime, DateOnly, and TimeOnly 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.

C#

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

Important

CriteriaOperator.FromLambda does not support all possible LINQ expressions as our criteria language and LINQ are not fully compatible. For instance, GroupBy, OrderBy, and other LINQ methods for filtering are not supported.