How to Dynamically Select Anonymous Types with Multiple Properties Using LINQ Expression Trees?

Use LINQ expression tree to dynamically select anonymous types

Introduction

LINQ expression trees provide a powerful mechanism for dynamically creating and modifying queries. A common need is to select an anonymous type with multiple properties. While selecting a single property is relatively simple, defining multiple properties in a select lambda can present challenges.

Solution using reflected emission

To solve this problem, we can use reflective emission and auxiliary classes to dynamically generate anonymous types. The following code demonstrates how to achieve this:

SelectDynamic method

<code class="language-csharp">public static IQueryable SelectDynamic(this IQueryable source, IEnumerable<string> fieldNames)
{
    // 创建属性名称和相应属性信息的字典
    Dictionary<string, PropertyInfo> sourceProperties = fieldNames.ToDictionary(name => name, name => source.ElementType.GetProperty(name));

    // 生成动态类型
    Type dynamicType = LinqRuntimeTypeBuilder.GetDynamicType(sourceProperties.Values);

    // 创建表达式树
    ParameterExpression sourceItem = Expression.Parameter(source.ElementType, "t");
    IEnumerable<MemberBinding> bindings = dynamicType.GetFields().Select(p => Expression.Bind(p, Expression.Property(sourceItem, sourceProperties[p.Name]))).OfType<MemberBinding>();

    Expression selector = Expression.Lambda(
        Expression.MemberInit(
            Expression.New(dynamicType.GetConstructor(Type.EmptyTypes)),
            bindings
        ),
        sourceItem
    );

    // 返回带有新select表达式的查询
    return source.Provider.CreateQuery(
        Expression.Call(
            typeof(Queryable),
            "Select",
            new Type[] { source.ElementType, dynamicType },
            Expression.Constant(source),
            selector
        )
    );
}</code>

LinqRuntimeTypeBuilder class

<code class="language-csharp">public static class LinqRuntimeTypeBuilder
{
    // ...
    public static Type GetDynamicType(IEnumerable<PropertyInfo> fields)
    {
        // ...
        string className = GetTypeKey(fields); //  修改参数类型

        TypeBuilder typeBuilder = moduleBuilder.DefineType(className, TypeAttributes.Public | TypeAttributes.Class | TypeAttributes.Serializable);

        foreach (var field in fields)                    
            typeBuilder.DefineField(field.Name, field.PropertyType, FieldAttributes.Public); // 使用field.Name 和 field.PropertyType

        return typeBuilder.CreateType();
    }
    // ...
}</code>

Example usage

To select an anonymous type with multiple properties, use the following syntax:

<code class="language-csharp">var v = from c in Countries
        where c.City == "London"
        select new { c.Name, c.Population };</code>

You can now access properties of anonymous types like any other instance:

<code class="language-csharp">Console.WriteLine(v.Name);
Console.WriteLine(v.Population);</code>

Note: The above code snippet needs to be supplemented with a complete implementation of the LinqRuntimeTypeBuilder class, including GetTypeKey and other helper methods that may be needed. The complete implementation is complex and requires handling various exceptions and type checking. This is just a simplified example to illustrate the core idea. In actual application, improvement and error handling need to be carried out according to specific needs. In addition, directly using reflected emission to build dynamic types may affect performance, and the pros and cons should be weighed based on the actual situation.

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