C#表达式目录树系列之2 --常见的表达式目录树的实例
概要
本文主要列举出一些常用的表达式目录树实例，便于以此为基础，构建更复杂的表达式目录树，以满足各种多变的业务场景。
表达式目录树的构建，除了使用Expression类提供的静态方法外，一般还需要同反射结合在一起。例如通过反射找到类中具体定义的方法，通过反射找到类中的构造函数等。
常用表达式目录树构建实例
Student类的定义详见附录。
访问对象的属性
构建目标 s => s.FirstName s是Student类型
代码如下：
ParameterExpression sParameter = Expression.Parameter(typeof(Student), "s");
MemberExpression firstNameExpression = Expression.Property(sParameter,typeof(Student).GetProperty("FirstName"));
LambdaExpression lengthLambdaExpression = Expression.Lambda<Func<Student,string>>(firstNameExpression,sParameter);
Console.WriteLine(lengthLambdaExpression);
var lengthDelegate = (Func<Student,string>) lengthLambdaExpression.Compile();
Console.WriteLine(lengthDelegate.Invoke(new Student("Tom","Green",14)));
代码执行流程为：
- 构建Lambda表达式中的变量s
- 构建表达式目录树s.FirstName
- 调用LambdaExpression的Lambda方法生成表达式： s => s.FirstName
- 调用LambdaExpression的Compile方法生成委托对象
- 委托对象调用Invoke方法，执行 s.FirstName代码。
访问嵌套对象属性
构建目标 s => s.FirstName.Length s是Student类型
ParameterExpression sParameter = Expression.Parameter(typeof(Student), "s");
MemberExpression firstNameExpression = Expression.Property(sParameter,typeof(Student).GetProperty("FirstName"));
MemberExpression lengthExpression = Expression.Property(firstNameExpression,typeof(string).GetProperty("Length"));
LambdaExpression lengthLambdaExpression = Expression.Lambda<Func<Student,int>>(lengthExpression,sParameter);
Console.WriteLine(lengthLambdaExpression);
var lengthDelegate = (Func<Student,int>) lengthLambdaExpression.Compile();
Console.WriteLine(lengthDelegate.Invoke(new Student("Tom","Green",14)));
访问对象的方法
构建目标： s => s.GetFullName() s是Student类型
ParameterExpression sParameter = Expression.Parameter(typeof(Student),"s");
MethodCallExpression getFullNameMethod = Expression.Call(
sParameter,
typeof(Student).GetMethod("GetFullName",new Type[]{}), // method
new Expression[] // Work method's parameter
{
}
);
LambdaExpression lambdaGetFullNameMethod = Expression.Lambda<Func<Student,string>>(getFullNameMethod,sParameter);
Console.WriteLine(lambdaGetFullNameMethod);
var lambdaGetFullNameDelegate = (Func<Student,string>) lambdaGetFullNameMethod.Compile();
Console.WriteLine(lambdaGetFullNameDelegate.Invoke(new Student("Tom", "Green", 13)));
访问对象方法（含参数）
构建目标：m => m.GetFullName(“Mr”) m是Student类型
ParameterExpression mParameter = Expression.Parameter(typeof(Student),"m");
string prefix = "Mr";
MethodCallExpression getFullNameMethod = Expression.Call(
mParameter,
typeof(Student).GetMethod("GetFullName", new Type[]{typeof(string)}), // method
new Expression[] // Work method's parameter
{
Expression.Constant(prefix)
}
);
LambdaExpression lambdaGetFullNameMethod = Expression.Lambda<Func<Student,string>>(getFullNameMethod,mParameter);
Console.WriteLine(lambdaGetFullNameMethod);
var lambdaGetFullNameDelegate = (Func<Student,string>) lambdaGetFullNameMethod.Compile();
Console.WriteLine(lambdaGetFullNameDelegate.Invoke(new Student("Tom", "Green", 13)));
访问类中静态方法
调用string类的Concat方法实现字符串拼接。
构建目标：c => Concat(c.FirstName, c.LastName) c是Student类型
ParameterExpression cParameter = Expression.Parameter(typeof(Student),"c");
MemberExpression firstNameExpression = Expression.Property(cParameter, typeof(Student).GetProperty("FirstName"));
MemberExpression lastNameExpression = Expression.Property(cParameter, typeof(Student).GetProperty("LastName"));
MethodCallExpression ConcatMethod = Expression.Call(
typeof(string).GetMethod("Concat", BindingFlags.Public | BindingFlags.Static, null, CallingConventions.Any, new Type[]{typeof(string),typeof(string)}, null), // method
new Expression[] // Work method's parameter
{
firstNameExpression,
lastNameExpression
}
);
LambdaExpression lambdaConcatMethod = Expression.Lambda<Func<Student,string>>(ConcatMethod,cParameter);
Console.WriteLine(lambdaConcatMethod);
var lambdaConcatDelegate = (Func<Student,string>) lambdaConcatMethod.Compile();
Console.WriteLine(lambdaConcatDelegate.Invoke(new Student("Tom", "Green", 13)));
调用类中含参数的构造方法创建实例
构建目标：(n, l, m) => new Student(n, l, m)
n,l为string类型,m是int类型
ParameterExpression n = Expression.Parameter(typeof(string),"n");
ParameterExpression l = Expression.Parameter(typeof(string),"l");
ParameterExpression m = Expression.Parameter(typeof(int),"m");
ConstructorInfo ctor = typeof(Student).GetConstructor(
BindingFlags.Instance | BindingFlags.Public,
null,
CallingConventions.HasThis,
new[] { typeof(string),typeof(string), typeof(int) }, new ParameterModifier[] {}
);
var expressionNew = Expression.New(ctor,new Expression[]{n,l,m});
LambdaExpression newLambda = Expression.Lambda<Func<string,string,int,Student>>(expressionNew, new ParameterExpression[]{n,l,m});
Console.WriteLine(newLambda);
var newDelegate = (Func<string,string,int,Student>) newLambda.Compile();
Console.WriteLine(newDelegate.Invoke("Tom","Green",13).GetFullName());
调用类中构造方法创建实例并给属性赋值
调用Student类的无参构造函数创建实例，并将已有Student实例中的属性值赋值给新创建的实例。
构建目标：r => new Student() {FirstName = r.FirstName, LastName = r.LastName, Age = r.Age} r为Student类型
Student stuResource = new Student("Jack", "Green",12);
ParameterExpression r = Expression.Parameter(typeof(Student),"r");
ConstructorInfo ctor = typeof(Student).GetConstructor(
BindingFlags.Instance | BindingFlags.Public,
null,
CallingConventions.HasThis,
new Type [] { }, new ParameterModifier[] {}
);
List<MemberBinding> bindings = new List<MemberBinding>();
foreach(PropertyInfo info in typeof(Student).GetProperties()){
MemberExpression rMember = Expression.Property(r,typeof(Student).GetProperty(info.Name));
MemberBinding binding = Expression.Bind(info,rMember);
bindings.Add(binding);
}
MemberInitExpression memberInitExpression = MemberInitExpression memberInitExpression = Expression.MemberInit( Expression.New(ctor, Array.Empty<Expression>()), bindings.ToArray());
LambdaExpression lambdaStudentNewExpression = Expression.Lambda<Func<Student,Student>>(memberInitExpression,r);
Console.WriteLine(lambdaStudentNewExpression);
var lambdaStudentNewDelegate = (Func<Student,Student>) lambdaStudentNewExpression.Compile();
Console.WriteLine(lambdaStudentNewDelegate.Invoke(stuResource).GetFullName());
嵌套调用方法
构建目标： c => Concat(c.GetFullName(), c.LastName) c为Student类型
ParameterExpression cParameter = Expression.Parameter(typeof(Student),"c");
MethodCallExpression getFullNameMethod = Expression.Call(
cParameter,
typeof(Student).GetMethod("GetFullName", new Type[]{}, null), // method
new Expression[] // Work method's parameter
{
}
);
MemberExpression lastNameExpression = Expression.Property(cParameter, typeof(Student).GetProperty("LastName"));
MethodCallExpression ConcatMethod = Expression.Call(
typeof(string).GetMethod("Concat", BindingFlags.Public | BindingFlags.Static, null, CallingConventions.Any, new Type[]{typeof(string),typeof(string)}, null), // method
new Expression[] // Work method's parameter
{
getFullNameMethod,
lastNameExpression
}
);
LambdaExpression lambdaConcatMethod = Expression.Lambda<Func<Student,string>>(ConcatMethod, cParameter);
Console.WriteLine(lambdaConcatMethod);
var lambdaConcatDelegate = (Func<Student,string>) lambdaConcatMethod.Compile();
Console.WriteLine(lambdaConcatDelegate.Invoke(new Student("Tom", "Green", 13)));
附录
public class Student {
public Student(string firstName, string lastName, int age){
this.FirstName = firstName;
this.LastName = lastName;
this.Age = age;
}
public Student(){
}
public string FirstName {get;set;}
public string LastName {get;set;}
public int Age {get;set;}
public string GetFullName(){
return FirstName + ", " + LastName;
}
public string GetFullName(string prefix){
return prefix + " " + FirstName + ", " + LastName;
}
}
表达式目录树系列全部文章
- C#表达式目录树系列之1 – 表达式目录树基本概念
- C#表达式目录树系列之2 – 常见的表达式目录树的实例
- C#表达式目录树系列之3 – 为EF查询实现动态OrderBy
- C#表达式目录树系列之4 – 解决C#泛型约束与无法创建带参数的泛型实例的矛盾
- C#表达式目录树系列之5 – 动态创建查询表达式