c# - 批量更新动态指定列

Question

我将使用github.com/loresoft/EntityFramework.Extended库进行批量更新。以下是用于批量更新的示例代码：

context
.Tasks
.Where(t => t.StatusId == 1)
.Update(t => new Task { StatusId = 2 }) 

我想指定用户指定的 StatusId 或其他属性，如 Discontinued、UnitsInStock。

我如何通过将属性作为字符串传递并构建动态表达式来实现这一点。

Answer 1

只要更新的是静态值（即不引用其他变量），您可以使用以下方法：

public static Expression<Func<T,T>> CreateNewObjectExpression<T>(Dictionary<string, object> props) {
    // set props
    var type = typeof(T);
    // This is the `new T` part of the expression
    var newExpr = Expression.New(type);
    // Convert the values to set to expressions
    var members = from prop in props
                  let val = Expression.Constant(prop.Value)
                  select new KeyValuePair<string, Expression>(prop.Key, val);
    // this is the full new T { prop1 = val1 ... }
    var initExpr = BindMembers(newExpr, members);

    // And now we add the parameter part `t => `
    return Expression.Lambda<Func<T, T>>(initExpr, Expression.Parameter(typeof(T)));
}

static MemberInitExpression BindMembers(NewExpression expr, IEnumerable<KeyValuePair<string, Expression>> assignments) {
    Type type = expr.Type;
    List<MemberBinding> bindings = new List<MemberBinding>();
    foreach (var pair in assignments) {
        MethodInfo info = type.GetProperty(pair.Key).GetSetMethod();
        MemberBinding binding = Expression.Bind(info, pair.Value);
        bindings.Add(binding);
    }
    return Expression.MemberInit(expr, bindings);
}

这些方法可以像这样使用：

var props = new Dictionary<string,object>();
// get the properties from the user instead, of course
props["StatusId"] = 2;
// Now get the expression:
var updateExpr = CreateNewObjectExpression<Task>(props);
// and apply it
context.Tasks.Where(t => t.StatusId == 1)
    .Update(updateExpr);

请注意仅设置目标类中存在且可写的属性，否则会崩溃。