它是这样的：

public static IQueryable<TSource> WhereEqual<TSource, TProperty>(this IQueryable<TSource> query, Expression<Func<TSource, TProperty>> propertySelector, TProperty value)
{
    var body2 = Expression.Equal(propertySelector.Body, Expression.Constant(value));
    var lambda = Expression.Lambda<Func<TSource, bool>>(body2, propertySelector.Parameters);
    return query.Where(lambda);
}

使用方法如下：

IQueryable<FuelCard> query = // Get cards from database as IQueryable

query = query.WhereEqual(x => x.Id, 85);
query = query.WhereEqual(x => x.User.LastName + " " + x.User.FirstName, "Jon Green");

一般来说，多个.Where()都隐含在&&中（在 * 和 * 之间），所以你只需要创建一个基于属性选择器表达式加上等于加上传递的值的表达式，然后返回一个使用该表达式的.Where()。

Xanatos已经完全回答了你的问题，但是我只是想演示一些其他的方法来概括你的数据库表和查询语句。第一种是使用函数+接口来代替lambda：

context.Users.Where(ReallyComplexExpression());
 private static Expression<Func<IHasName, bool>> ReallyComplexExpression()
    {
        return x =>
            x.FirstName != null && x.LastName == "Salmon" || x.FirstName == "Fish" && x.LastName == "hermonie" ||
            x.FirstName == "Thanks" && x.LastName == "for the fish";
    }

另一种方法是泛型单例或泛型静态函数，它们确实倾向于与Linq -〉SQL一起工作，并且通常以更可读的方式结束
这是十分粗糙而不冉然的但却表明了基本的概念：

public interface IHasId
{
    int Id { get; set; }
}
public interface IHasName
{
    string FirstName { get;  }
    string LastName { get;  }

}
public class Employee : IHasName, IHasId
{
    [Column("Forename")] //this ensures it is still called "Forename" in the database
    public string FirstName { get; set; }
    [Column("Surname")]
    public string LastName { get; set; }
    [Key]
    [Column("EmployeeId")]//this ensures you are following DB best practice through the same technique as above
    public int Id { get; set; }
}
public class User : IHasName, IHasId
{
    public string FirstName { get; set; }
    public string LastName { get; set; }
    [Key]
    [Column("UserId")]
    public int Id { get; set; }

    //equally you could add
    [notmapped]
    public string FullName => $"{FirstName} {LastName}"; 
 //or add this as an extension function for all IHasName
}

public class Job: IHasName, IHasId
{
    public string Name { get; set; }

    [NotMapped] //not mapped tells entity framework you dont want these fields in the DB, so in the DB their would only be "name" but in your code, firstname & lastname are pseudonyms for Name 
    public string FirstName => Name;

    [NotMapped]
    public string LastName => Name;
    [Key]
    [Column("JobId")]
    public int Id { get; set; }

}

public class ModelContext : DbContext
{
    public DbSet<Employee> Employees { get; set; }
    public DbSet<User> Users { get; set; }
    public DbSet<Job> Jobs { get; set; }
}

public static class GenericExtensions
{

    public static IQueryable<TModel> GetById<TModel>(this IQueryable<TModel> db, int Id) where TModel : class, IHasId
    {
        return db.Where(x => x.Id == Id);
    }
    public static IQueryable<TModel> GetByAnyName<TModel>(this IQueryable<TModel> db, string name) where TModel : class, IHasName
    {
        return db.Where(x => x.FirstName == name || x.LastName == name);
    }
    public static IQueryable<TModel> GetByFirstAndLastName<TModel>(this IQueryable<TModel> db, string firstName, string lastName) where TModel : class, IHasName
    {
        return db.Where(x => x.FirstName == firstName && x.LastName == lastName);
    }
}

class Program
{
    static void Main(string[] args)
    {
        ModelContext context = new ModelContext();

        var filteredEmpl =context.Employees.GetByAnyName("John").GetById(1);

        var filteredUsers =context.Users.GetByFirstAndLastName("Terry","Richards").GetById(1);

         var filteretJobs =context.Jobs.GetByAnyName("Supervisor").GetById(1); //this wouldn't work as "Name" is the DB column, so Linq->SQL would fail when confronted with "FirstName" 
    }
}

这听起来和我解决的一个问题非常相似。在我的例子中，我必须根据用Sql语法定义的复杂过滤器将对象过滤到不同的类别中。
我已经创建了一个Nuget包DynamicFilter.Sql来从Sql动态生成lambda表达式。
以下是一个示例

var filter = FilterExpression.Compile<User>("(Age > 10) and (Name like 'Alice%' or Name like 'Bob%')");

//Filter a collection
var filteredUsers = users.Where(filter);

该软件包是开源的，可在github上获得。