使用EF Core 2.1检查表是否存在

Question

在Entity Framework中,可以通过以下方式检查表的存在:

bool exists = context.Database
                 .SqlQuery<int?>(@"
                     SELECT 1 FROM sys.tables AS T
                     INNER JOIN sys.schemas AS S ON T.schema_id = S.schema_id
                     WHERE S.Name = 'SchemaName' AND T.Name = 'TableName'")
                 .SingleOrDefault() != null;

我使用EF Core 2.1并且该方法SqlQuery不存在.

检查表是否存在的正确方法是什么？理想情况下,无需尝试访问该表,并假设在抛出异常时它不存在.

编辑:我最后的实施

public bool TableExists(string tableName)
{
    return TableExists("dbo", tableName);
}

public bool TableExists(string schema, string tableName)
{
    var connection = Context.Database.GetDbConnection();

    if (connection.State.Equals(ConnectionState.Closed))
        connection.Open();

    using (var command = connection.CreateCommand())
    {
        command.CommandText = @"
            SELECT 1 FROM INFORMATION_SCHEMA.TABLES 
            WHERE TABLE_SCHEMA = @Schema
            AND TABLE_NAME = @TableName";

        var schemaParam = command.CreateParameter();
        schemaParam.ParameterName = "@Schema";
        schemaParam.Value = schema;
        command.Parameters.Add(schemaParam);

        var tableNameParam = command.CreateParameter();
        tableNameParam.ParameterName = "@TableName";
        tableNameParam.Value = tableName;
        command.Parameters.Add(tableNameParam);

        return command.ExecuteScalar() != null;
    }
}

Answer 1

有ExecuteSqlCommand.

context.Database.ExecuteSqlCommand("...")

但是,它仅限于返回一个整数,表示受影响的行数.如果您正在进行操作SELECT,则不会影响任何行,因此它并不能真正满足您的需求.

还有FromSql,但这只适用于表,而不适用于数据库级别:

context.TableName.FromSql("SELECT ...")

对于你正在做的事情,一个更好的选择是DbConnection从EF 获得,并创建自己的DbCommand.这会返回您的期望:

var conn = context.Database.GetDbConnection();
if (conn.State.Equals(ConnectionState.Closed)) await conn.OpenAsync();
using (var command = conn.CreateCommand()) {
    command.CommandText = @"
    SELECT 1 FROM sys.tables AS T
        INNER JOIN sys.schemas AS S ON T.schema_id = S.schema_id
    WHERE S.Name = 'SchemaName' AND T.Name = 'TableName'";
    var exists = await command.ExecuteScalarAsync() != null;
}

这里提醒一句:千万不要把DbConnection你从GetDbConnection()一个using声明,或关闭它,当你完成.该连接用于该DbContext实例的生命周期.因此,如果您关闭它,EF发出的任何后续请求都将失败.它也可能已经在你的代码之前打开了,这就是为什么我在那里放一个测试,看它是否在调用之前关闭了OpenAsync().