ConstantExpression 方法

ConstantExpression 必须与 MemberExpression 的类型相同。此示例中的值是一个字符串，在创建 ConstantExpression 实例之前将其转换。

private static ConstantExpression GetConstant(Type type, string value)
{
    // Discover the type, convert it, and create ConstantExpression 
    ConstantExpression constant = null;
    if (type == typeof(int))
    {
        int num;
        int.TryParse(value, out num);
        constant = Expression.Constant(num);
    }
    else if(type == typeof(string))
    {
        constant = Expression.Constant(value);
    }
    else if (type == typeof(DateTime))
    {
        DateTime date;
        DateTime.TryParse(value, out date);
        constant = Expression.Constant(date);
    }
    else if (type == typeof(bool))
    {                
        bool flag;
        if (bool.TryParse(value, out flag))
        {
            flag = true;
        }
        constant = Expression.Constant(flag);
    }
    else if (type == typeof(decimal))
    {
        decimal number;
        decimal.TryParse(value, out number);
        constant = Expression.Constant(number);
    }
    return constant;
}