使用Dapper和SqlCE进行开发的时候，如果数据库的某字段是采用的ntext数据类型，并且在这个字段存储的数据超过了4000个字符，会报如下的错误：
Invalid parameter Size value ‘-1‘. The value must be greater than or equal to 0.
在Google上以“dapper sqlce ntext”作为关键词搜索，可以找到如下两个解决办法：
1. Inserting a string larger then 4000 characters using Sql CE 4.0
2. Attempting to Modify Dapper to Support SQL Server CE‘s ntext type
这两个解决办法，要么通用性较差，要么使用比较麻烦。
通过查看Dapper的源码，发现Dapper在构造参数的动态方法中针对实体类属性为DbString的类型和数据类型为DbType.Xml进行了特别处理，我们也可以在这里入手，针对长字符串进行特别处理。
解决办法：
一、新建一个Attribute，用来标记需要特别处理的实体类属性；
[AttributeUsage(AttributeTargets.Property)]
public class LongStringAttribute : Attribute
{
}
二、新建一个类，用来处理添加NText类型参数；
public class LongString
{
public static void AddParameter(IDbCommand command, string name, string value)
{
var param = command.CreateParameter();
param.ParameterName = name;
param.Value = (object)value ?? DBNull.Value;
param.DbType = DbType.String;
int length = -1;
if (!string.IsNullOrEmpty(value))
length = value.Length;
if (length == -1 && value != null && value.Length <= 4000)
{
param.Size = 4000;
}
else
{
param.Size = length;
}
if (value != null)
{
if (length > 4000 && param.GetType().Name == "SqlCeParameter")
{
param.GetType().GetProperty("SqlDbType").SetValue(param, SqlDbType.NText, null);
param.Size = length;
}
}
command.Parameters.Add(param);
}
}
三、修改Dapper的源码，在SqlMapper.CreateParamInfoGenerator方法中，找到以下代码
if (prop.PropertyType == typeof(DbString))
{
il.Emit(OpCodes.Ldloc_0); // stack is now [parameters] [typed-param]
il.Emit(OpCodes.Callvirt, prop.GetGetMethod()); // stack is [parameters] [dbstring]
il.Emit(OpCodes.Ldarg_0); // stack is now [parameters] [dbstring] [command]
il.Emit(OpCodes.Ldstr, prop.Name); // stack is now [parameters] [dbstring] [command] [name]
il.EmitCall(OpCodes.Callvirt, typeof(DbString).GetMethod("AddParameter"), null); // stack is now [parameters]
continue;
}
//我们的代码插入到这里
DbType dbType = LookupDbType(prop.PropertyType, prop.Name);
if (dbType == DbType.Xml)
{
// this actually represents special handling for list types;
il.Emit(OpCodes.Ldarg_0); // stack is now [parameters] [command]
il.Emit(OpCodes.Ldstr, prop.Name); // stack is now [parameters] [command] [name]
il.Emit(OpCodes.Ldloc_0); // stack is now [parameters] [command] [name] [typed-param]
il.Emit(OpCodes.Callvirt, prop.GetGetMethod()); // stack is [parameters] [command] [name] [typed-value]
if (prop.PropertyType.IsValueType)
{
il.Emit(OpCodes.Box, prop.PropertyType); // stack is [parameters] [command] [name] [boxed-value]
}
il.EmitCall(OpCodes.Call, typeof(SqlMapper).GetMethod("PackListParameters"), null); // stack is [parameters]
continue;
}
修改之后的代码如下：
if (prop.PropertyType == typeof(DbString))
{
il.Emit(OpCodes.Ldloc_0); // stack is now [parameters] [typed-param]
il.Emit(OpCodes.Callvirt, prop.GetGetMethod()); // stack is [parameters] [dbstring]
il.Emit(OpCodes.Ldarg_0); // stack is now [parameters] [dbstring] [command]
il.Emit(OpCodes.Ldstr, prop.Name); // stack is now [parameters] [dbstring] [command] [name]
il.EmitCall(OpCodes.Callvirt, typeof(DbString).GetMethod("AddParameter"), null); // stack is now [parameters]
continue;
}
//这里插入修改的代码
Attribute lStrAttr = Attribute.GetCustomAttribute(prop, typeof(LongStringAttribute));
if (lStrAttr != null)
{
//special handling for long string
il.Emit(OpCodes.Ldarg_0); // stack is now [parameters] [command]
il.Emit(OpCodes.Ldstr, prop.Name); // stack is now [parameters] [command] [name]
il.Emit(OpCodes.Ldloc_0); // stack is now [parameters] [command] [name] [typed-param]
il.Emit(OpCodes.Callvirt, prop.GetGetMethod()); // stack is [parameters] [command] [name] [string]
il.EmitCall(OpCodes.Call, typeof(LongString).GetMethod("AddParameter"), null);
continue;
}
DbType dbType = LookupDbType(prop.PropertyType, prop.Name);
if (dbType == DbType.Xml)
{
// this actually represents special handling for list types;
il.Emit(OpCodes.Ldarg_0); // stack is now [parameters] [command]
il.Emit(OpCodes.Ldstr, prop.Name); // stack is now [parameters] [command] [name]
il.Emit(OpCodes.Ldloc_0); // stack is now [parameters] [command] [name] [typed-param]
il.Emit(OpCodes.Callvirt, prop.GetGetMethod()); // stack is [parameters] [command] [name] [typed-value]
if (prop.PropertyType.IsValueType)
{
il.Emit(OpCodes.Box, prop.PropertyType); // stack is [parameters] [command] [name] [boxed-value]
}
il.EmitCall(OpCodes.Call, typeof(SqlMapper).GetMethod("PackListParameters"), null); // stack is [parameters]
continue;
}
这样就可以了，使用的时候，如果我们数据表的某字段是ntext类型，那么我们只需要在定义相应实体类的时候，给相应属性加上LongStringAttribute，Dapper就可以自动识别这个字段，插入正确的数据了。
使用代码如下：
//实体类定义
using System;
using Dapper;
namespace Entity
{
public class product
{
public int Id { get; set; }
public int shopid { get; set; }
public string type { get; set; }
public string outid { get; set; }
public string link { get; set; }
public string title { get; set; }
[LongString]
public string content { get; set; }
public decimal price { get; set; }
public int amount { get; set; }
}
}
//调用
product p = new product();
p.shopid = 1;
p.title = "梁振英:\"占中\"者不要试探北京忍耐底线";
p.link = "http://news.163.com/14/1021/13/A936JGST0001124J.html";
string str = FileHelper.ReadTextFile("content.txt");
p.content = str;
p.type = "netease";
p.outid = "A936JGST0001124J";
p.price = 123.45M;
p.amount = 999;
SqlCeConnection conn = new SqlCeConnection("Data Source=test.sdf");
conn.Open();
string sql = "insert into products(shopid,type,outid,link,title,content,price,amount) values(@shopid,@type,@outid,@link,@title,@content,@price,@amount)";
SqlMapper.Execute(conn, sql, p);
conn.Close();
我使用的Dapper版本是博客园里@wushilonng改写的针对.NET 2.0的版本，未知最新版Dapper是否针对这方面做了改进。
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时间： 2024-10-18 02:53:13