.NET 中的表达式树如何动态构建查询？

<font color="#006699">public class Person {
    public string Name { get; set; }
    public int Age { get; set; }
}</font>

<font color="#006699">public Expression<Func<Person, bool>> BuildExpression(string propertyName, object value)
{
    var param = Expression.Parameter(typeof(Person), "c");
    var property = Expression.Property(param, propertyName);
    var constant = Expression.Constant(value);
    var equality = Expression.Equal(property, constant);

    return Expression.Lambda<Func<Person, bool>>(equality, param);
}</font>

<font color="#006699">var expr = BuildExpression("Name", "Tom");
var people = dbContext.People.Where(expr).ToList();</font>

<font color="#006699">var param = Expression.Parameter(typeof(Person), "c");

// c.Age > 18
var ageProp = Expression.Property(param, "Age");
var ageCond = Expression.GreaterThan(ageProp, Expression.Constant(18));

// c.Name == "Tom"
var nameProp = Expression.Property(param, "Name");
var nameCond = Expression.Equal(nameProp, Expression.Constant("Tom"));

// c.Age > 18 && c.Name == "Tom"
var andExpr = Expression.AndAlso(ageCond, nameCond);

var lambda = Expression.Lambda<Func<Person, bool>>(andExpr, param);</font>

<font color="#006699">dbContext.People.Where(lambda).ToList();</font>