C# 为函数指定的参数值无效。[参数=1，函数名称（如果已知）=isnull]

我想从列与给定参数匹配的表中进行选择。如果参数为null，我想从表中选择所有记录。下面的相关代码就是引发此错误的原因

    private static string _dbJobCreate = 
"CREATE TABLE Job (jID int primary key identity(1,1), jAddress nvarchar(64) not null);";

    private static string _dbJobSelect = 
"SELECT jID FROM Job WHERE jAddress = @jAddress OR @jAddress IS NULL";

    public static DataTable GetJobs(string jAddress)
    {
        SqlCeParameter pjAddress = new SqlCeParameter();
        pjAddress.ParameterName = "@jAddress";

        if (!string.IsNullOrEmpty(jAddress))
        {
            pjAddress.Value = jAddress;
        }
        else
        {
            pjAddress.Value = DBNull.Value;
        }

        return ExecuteDataTable(_dbJobSelect, pjAddress);
    }

异常：为函数指定的参数值无效。[参数=1，函数名如果已知=isnull]

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如何在SQLCE中高效地完成这项工作而不出错？

我的解决方案是从数据库中选择所有行，并在传入参数时过滤.NET中的行。如果有大量的工作，这可能会变得很麻烦，尽管我想如果发生这种情况，我会转移到一个真正的数据库

private static string _dbJobSelect = "SELECT jID, jAddress FROM Job";

public static DataTable GetJobs(string jAddress)
{
    DataTable dt = ExecuteDataTable(_dbJobSelect);

    if (!string.IsNullOrEmpty(jAddress))
    {
        DataView dv = dt.DefaultView;
        dv.RowFilter = string.Format("jAddress = '{0}'", jAddress);
        dt = dv.ToTable();
    }

    return dt;
}

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通过指定传递给查询的参数类型，可以避免此错误。因此，您需要做的就是：

pjAddress.DbType = DbType.String;

您可以将其转换为varchar

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我最近遇到了这个问题，发现我实际上没有将查询参数添加到IDB命令中。我不确定ExecuteDataTable方法是什么样子，但我的代码类似于以下代码，其中db是IDbConnection实例：

var sql = "SELECT jID FROM Job WHERE jAddress = @jAddress OR @jAddress IS NULL";
var cmd = db.CreateCommand();
cmd.CommandText = sql;
var param = cmd.CreateParameter();
param.DbType = DbType.String;
param.ParameterName = "@jAddress";
param.Value = string.IsNullOrEmpty(pjAddress) ? DBNull.Value : (object)pjAddress;
cmd.Parameters.Add(param);  // THIS WAS THE STEP I WAS MISSING!!!
// ... rest of the code to execute the query and load the results ...

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添加cmd.Parameters.Addparam后，异常消失。

有用的讨论：对我来说，这是一个完美的解决方案。

var sql = "SELECT jID FROM Job WHERE jAddress = @jAddress OR @jAddress IS NULL";
var cmd = db.CreateCommand();
cmd.CommandText = sql;
var param = cmd.CreateParameter();
param.DbType = DbType.String;
param.ParameterName = "@jAddress";
param.Value = string.IsNullOrEmpty(pjAddress) ? DBNull.Value : (object)pjAddress;
cmd.Parameters.Add(param);  // THIS WAS THE STEP I WAS MISSING!!!
// ... rest of the code to execute the query and load the results ...