private Func <IDataReader, T> CompileMapperFunction()
{
var exps = new List <Expression>();
var dataReaderExprParam = Expression.Parameter(typeof(IDataRecord), "datareader");
var targetExp = Expression.Variable(typeof(T));
//var target = new T();
exps.Add(Expression.Assign(targetExp, Expression.New(targetExp.Type)));
//does int based lookup
var indexerInfo = typeof(IDataRecord).GetProperty("Item", new[] { typeof(int) });
var columnNames = Enumerable.Range(0, _dataReader.FieldCount)
.Select(i => new { index = i, name = _dataReader.GetName(i) });
var properties = typeof(T).GetProperties(BindingFlags.Public | BindingFlags.Instance);
foreach (var column in columnNames)
{
var property = AttributeExtension.GetUnderlyingPropertyByName(properties, column.name);
if (property == null)
{
continue;
}
var readerIndex = Expression.Constant(column.index);
// equivalent to datareader[(int)readerIndex] where datareader is incoming parameter.
var valueExpr = Expression.MakeIndex(dataReaderExprParam, indexerInfo, new[] { readerIndex });
var actualType = _dataReader.GetFieldType(column.index);
Expression safeCastExpression;
//if property type in model doesn't match the underlying type in SQL, we first convert into actual SQL type.
if (actualType != property.PropertyType)
{
safeCastExpression = Expression.Convert(valueExpr, actualType);
}
//otherwise we do nothing.
else
{
safeCastExpression = valueExpr;
}
var isReaderDbNull = Expression.Call(dataReaderExprParam, "IsDBNull", null, readerIndex);
var propertyExpression = Expression.Property(targetExp, property);
/*
* if(datareader.IsDBNull((int)readerIndex){
* target.property = default;
* }else{
* target.property = (castType)value;
* }
*/
var assignmentBlock = Expression.Condition(
Expression.IsTrue(isReaderDbNull),
Expression.Assign(propertyExpression, Expression.Default(property.PropertyType)),
Expression.Assign(propertyExpression, Expression.Convert(safeCastExpression, property.PropertyType)
)
);
exps.Add(assignmentBlock);
}
//return target;
exps.Add(targetExp);
var func = Expression.Lambda <Func <IDataReader, T> >(
Expression.Block(new[] { targetExp }, exps), dataReaderExprParam);
return(func.Compile());
}
