Chapter85:GenericLambdaQueryBuilder

Section85.1:QueryFilterclass

Thisclassholdspredicatefiltersvalues.

publicclassQueryFilter

{

publicstringPropertyName{get;set;}

publicstringValue{get;set;}

publicOperatorOperator{get;set;}

//Inthequery{a=>a.Name.Equals(“Pedro”)}

//Propertynametofilter-propertyName=”Name”

//Filtervalue-value=”Pedro”

//Operationtoperform-operation=enumOperator.Equals

publicQueryFilter(stringpropertyName,stringvalue,OperatoroperatorValue)

{

PropertyName=propertyName;

Value=value;

Operator=operatorValue;

}

}

Enumtoholdtheoperationsvalues:

publicenumOperator

{

Contains, GreaterThan,

GreaterThanOrEqual,

LessThan,

LessThanOrEqualTo,

StartsWith,EndsWith,

Equals,

NotEqual

}

Section85.2:GetExpressionMethod

publicstaticExpression<Func<T,bool>>GetExpression<T>(IList<QueryFilter>filters)

{

Expressionexp=null;

//Representsanamedparameterexpression.{parm=>parm.Name.Equals()},itistheparampart

//TocreateaParameterExpressionneedthetypeoftheentitythatthequeryisagainstana

name

//ThetypeispossibletofindwiththegenericTandthenameisfixedparm

ParameterExpressionparam=Expression.Parameter(typeof(T),”parm”);

//Itisgoodparcticenevertrustintheclient,soitiswisetovalidate.

if(filters.Count==0)

returnnull;

//Theexpressioncreationdifferifthereisone,twoormorefilters.

if(filters.Count!=1)

{

if(filters.Count==2)

//Itisresultfromdirectcall.

//Forsimplicitysaketheprivateoverloadswillbeexplainedinanotherexample.

exp=GetExpression<T>(param,filters[0],filters[1]);

else

{

//Asthereisnomethodformorethantwofilters,

//IiteratethroughallthefiltersandputIinthequerytwoatatime

while(filters.Count>0)

{

//Retrievethefirsttwofilters

varf1=filters[0];

varf2=filters[1];

//TobuildaexpressionwithaconditionalANDoperationthatevaluates

//thesecondoperandonlyifthefirstoperandevaluatestotrue.

//ItneededtousetheBinaryExpressionaExpressionderivedclass

//ThathastheAndAlsomethodthatjointwoexpressiontogether

exp=exp==null?GetExpression<T>(param, filters[0], filters[1]):

Expression.AndAlso(exp,GetExpression<T>(param, filters[0], filters[1]));

nextfilters

//Removethetwojustusedfilters,forthemethodinthenextiterationfindsthe

filters.Remove(f1); filters.Remove(f2);

//Ifitisthatlastfilter,addthelastoneandremoveit

if(filters.Count==1)

{

exp=Expression.AndAlso(exp,GetExpression<T>(param,filters[0])); filters.RemoveAt(0);

}

}

}

}

else

//Itisresultfromdirectcall.

exp=GetExpression<T>(param,filters[0]);

//convertstheExpressionintoLambdaandretunsthequery

returnExpression.Lambda<Func<T,bool>>(exp,param);

}

Section85.3:GetExpressionPrivateoverload

Forone filter:

Hereiswherethequeryiscreated,itreceivesaexpressionparameterandafilter.

privatestaticExpressionGetExpression<T>(ParameterExpressionparam,QueryFilterqueryFilter)

{

//Representsaccessingafieldorproperty,sohereweareaccessingforexample:

//theproperty“Name”oftheentity

MemberExpressionmember=Expression.Property(param,queryFilter.PropertyName);

//Representsanexpressionthathasaconstantvalue,sohereweareaccessingforexample:

//thevaluesoftheProperty”Name”.

//AlsoforclaritysaketheGetConstantwillbeexplainedinanotherexample.

ConstantExpressionconstant=GetConstant(member.Type,queryFilter.Value);

//Withthesetwo,nowIcanbuildtheexpression

//everyoperatorhasitonewaytocall,sotheswitchwilldo.

switch(queryFilter.Operator)

{

caseOperator.Equals:

returnExpression.Equal(member,constant);

caseOperator.Contains:

returnExpression.Call(member,ContainsMethod,constant);

caseOperator.GreaterThan:

returnExpression.GreaterThan(member,constant);

caseOperator.GreaterThanOrEqual:

returnExpression.GreaterThanOrEqual(member,constant);

caseOperator.LessThan:

returnExpression.LessThan(member,constant);

caseOperator.LessThanOrEqualTo:

returnExpression.LessThanOrEqual(member,constant);

caseOperator.StartsWith:

returnExpression.Call(member,StartsWithMethod,constant);

caseOperator.EndsWith:

returnExpression.Call(member,EndsWithMethod,constant);

}

returnnull;

}

Fortwofilters:

ItreturnstheBinaryExpresioninstanceinsteadofthesimpleExpression.

privatestaticBinaryExpressionGetExpression<T>(ParameterExpressionparam,QueryFilterfilter1, QueryFilterfilter2)

{

//Builttwoseparatedexpressionandjointhemafter.

Expressionresult1=GetExpression<T>(param,filter1);

Expressionresult2=GetExpression<T>(param,filter2);

returnExpression.AndAlso(result1,result2);

}

Section85.4:ConstantExpressionMethod

ConstantExpressionmustbethesametypeoftheMemberExpression.Thevalueinthisexampleisastring,whichis converted before creating the ConstantExpressioninstance.

privatestaticConstantExpressionGetConstant(Typetype,stringvalue)

{

//Discoverthetype,convertit,andcreateConstantExpression

ConstantExpressionconstant=null;

if(type==typeof(int))

{

intnum;

int.TryParse(value,outnum);

constant=Expression.Constant(num);

}

elseif(type==typeof(string))

{

constant=Expression.Constant(value);

}

elseif(type==typeof(DateTime))

{

DateTimedate;

DateTime.TryParse(value,outdate);

constant=Expression.Constant(date);

}

elseif(type==typeof(bool))

{

boolflag;

if(bool.TryParse(value,outflag))

{

flag=true;

}

constant=Expression.Constant(flag);

}

elseif(type==typeof(decimal))

{

decimalnumber;decimal.TryParse(value,outnumber);

constant=Expression.Constant(number);

}

returnconstant;

}

Section85.5:Usage

Collectionfilters=newList();QueryFilterfilter=newQueryFilter(“Name”,”Burger”,Operator.StartsWith); filters.Add(filter);

Expression<Func<Food,bool>>query=ExpressionBuilder.GetExpression<Food>(filters);

Inthiscase,itisaqueryagainsttheFoodentity,thatwanttofindallfoodsthatstartwith”Burger”inthename.

Output:

query={parm=>a.parm.StartsWith(“Burger”)}

Expression<Func<T,bool>>GetExpression<T>(IList<QueryFilter>filters)