On Dapper's documentation found here it states:
Limitations and caveats
Dapper caches information about every query it runs, this allow it to materialize objects quickly and process parameters quickly. The current implementation caches this information in a ConcurrentDictionary object.
What exactly does this mean? Ex: Is it caching returned data, or the query itself, or bits of both?
It also says that "this [cached] data is never flushed". How does this effect the "cached information" if the design schema of the table(s) you are querying is changed?
As far as I know has each query you issue an Identity
, depending on the sql query, its command type and its parameters. The cache is a dictionary with concurrent access.
Dictionary<Identity, CacheInfo> _queryCache
This CacheInfo
object contains the IDataReader
and IDBCommand
functions and some control counters which limit the cached amount.
Since no server-side (database schema etc.) are cached, it actually doesn't have any influence.
Edit: Thats how the Identity class looks like used for caching.
private Identity(string sql, CommandType? commandType, string connectionString, Type type, Type parametersType, Type[] otherTypes, int gridIndex)
{
this.sql = sql;
this.commandType = commandType;
this.connectionString = connectionString;
this.type = type;
this.parametersType = parametersType;
this.gridIndex = gridIndex;
unchecked
{
hashCode = 17; // we *know* we are using this in a dictionary, so pre-compute this
hashCode = hashCode * 23 + commandType.GetHashCode();
hashCode = hashCode * 23 + gridIndex.GetHashCode();
hashCode = hashCode * 23 + (sql == null ? 0 : sql.GetHashCode());
hashCode = hashCode * 23 + (type == null ? 0 : type.GetHashCode());
if (otherTypes != null)
{
foreach (var t in otherTypes)
{
hashCode = hashCode * 23 + (t == null ? 0 : t.GetHashCode());
}
}
hashCode = hashCode * 23 + (connectionString == null ? 0 : connectionString.GetHashCode());
hashCode = hashCode * 23 + (parametersType == null ? 0 : parametersType.GetHashCode());
}
}
And here's the CacheInfo
class CacheInfo
{
public Func<IDataReader, object> Deserializer { get; set; }
public Func<IDataReader, object>[] OtherDeserializers { get; set; }
public Action<IDbCommand, object> ParamReader { get; set; }
private int hitCount;
public int GetHitCount() { return Interlocked.CompareExchange(ref hitCount, 0, 0); }
public void RecordHit() { Interlocked.Increment(ref hitCount); }
}
And finally the container of the cache.
static readonly System.Collections.Concurrent.ConcurrentDictionary<Identity, CacheInfo> _queryCache = new System.Collections.Concurrent.ConcurrentDictionary<Identity, CacheInfo>();
Have a look at the source code, its very well written and easy to follow / debug. Just drag the file into your project.
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