[转载][原创].NET 业务框架开发实战之十 第一阶段总结，深入浅出，水到渠成（前篇）

IQueryable CreateQuery(Expression expression);

IQueryable<TElement> CreateQuery<TElement>(Expression expression);

object Execute(Expression expression);

TResult Execute<TResult>(Expression expression);

}

public Query(QueryProvider provider) {
if (provider == null) {
throw new ArgumentNullException(“provider“);
}
this.provider = provider;
this.expression = Expression.Constant(this);
}

public Query(QueryProvider provider, Expression expression) {
if (provider == null) {
throw new ArgumentNullException(“provider“);
}
if (expression == null) {
throw new ArgumentNullException(“expression“);
}
if (!typeof(IQueryable<T>).IsAssignableFrom(expression.Type)) {
throw new ArgumentOutOfRangeException(“expression“);
}
this.provider = provider;
this.expression = expression;
}

Expression IQueryable.Expression {
get { return this.expression; }
}

Type IQueryable.ElementType {
get { return typeof(T); }
}

IQueryProvider IQueryable.Provider {
get { return this.provider; }
}

public IEnumerator<T> GetEnumerator() {
return ((IEnumerable<T>)this.provider.Execute(this.expression)).GetEnumerator();
}

IEnumerator IEnumerable.GetEnumerator() {
return ((IEnumerable)this.provider.Execute(this.expression)).GetEnumerator();
}

public override string ToString() {
return this.provider.GetQueryText(this.expression);
}
}

IQueryable<S> IQueryProvider.CreateQuery<S>(Expression expression) {
return new Query<S>(this, expression);
}

IQueryable IQueryProvider.CreateQuery(Expression expression) {
Type elementType = TypeSystem.GetElementType(expression.Type);
try {
return (IQueryable)Activator.CreateInstance(typeof(Query<>).MakeGenericType(elementType), new object[] { this, expression });
}
catch (TargetInvocationException tie) {
throw tie.InnerException;
}
}

S IQueryProvider.Execute<S>(Expression expression) {
return (S)this.Execute(expression);
}

object IQueryProvider.Execute(Expression expression) {
return this.Execute(expression);
}

public abstract string GetQueryText(Expression expression);
public abstract object Execute(Expression expression);
}

internal QueryTranslator() {
}

internal string Translate(Expression expression) {
this.sb = new StringBuilder();
this.Visit(expression);
return this.sb.ToString();
}

private static Expression StripQuotes(Expression e) {
while (e.NodeType == ExpressionType.Quote) {
e = ((UnaryExpression)e).Operand;
}
return e;
}

protected override Expression VisitMethodCall(MethodCallExpression m) {
if (m.Method.DeclaringType == typeof(Queryable) && m.Method.Name == “Where“) {
sb.Append(“SELECT * FROM (“);
this.Visit(m.Arguments[0]);
sb.Append(“) AS T WHERE “);
LambdaExpression lambda = (LambdaExpression)StripQuotes(m.Arguments[1]);
this.Visit(lambda.Body);
return m;
}
throw new NotSupportedException(string.Format(“The method ‘{0}’ is not supported“, m.Method.Name));
}

protected override Expression VisitUnary(UnaryExpression u) {
switch (u.NodeType) {
case ExpressionType.Not:
sb.Append(“ NOT “);
this.Visit(u.Operand);
break;
default:
throw new NotSupportedException(string.Format(“The unary operator ‘{0}’ is not supported“, u.NodeType));
}
return u;
}

protected override Expression VisitBinary(BinaryExpression b) {
sb.Append(“(“);
this.Visit(b.Left);
switch (b.NodeType) {
case ExpressionType.And:
sb.Append(“ AND “);
break;
case ExpressionType.Or:
sb.Append(“ OR“);
break;
case ExpressionType.Equal:
sb.Append(“ = “);
break;
case ExpressionType.NotEqual:
sb.Append(“ <> “);
break;
case ExpressionType.LessThan:
sb.Append(“ < “);
break;
case ExpressionType.LessThanOrEqual:
sb.Append(“ <= “);
break;
case ExpressionType.GreaterThan:
sb.Append(“ > “);
break;
case ExpressionType.GreaterThanOrEqual:
sb.Append(“ >= “);
break;
default:
throw new NotSupportedException(string.Format(“The binary operator ‘{0}’ is not supported“, b.NodeType));
}
this.Visit(b.Right);
sb.Append(“)“);
return b;
}

protected override Expression VisitConstant(ConstantExpression c) {
IQueryable q = c.Value as IQueryable;
if (q != null) {
// assume constant nodes w/ IQueryables are table references
sb.Append(“SELECT * FROM “);
sb.Append(q.ElementType.Name);
}
else if (c.Value == null) {
sb.Append(“NULL“);
}
else {
switch (Type.GetTypeCode(c.Value.GetType())) {
case TypeCode.Boolean:
sb.Append(((bool)c.Value) ? 1 : 0);
break;
case TypeCode.String:
sb.Append(“‘“);
sb.Append(c.Value);
sb.Append(“‘“);
break;
case TypeCode.Object:
throw new NotSupportedException(string.Format(“The constant for ‘{0}’ is not supported“, c.Value));
default:
sb.Append(c.Value);
break;
}
}
return c;
}

protected override Expression VisitMemberAccess(MemberExpression m) {
if (m.Expression != null && m.Expression.NodeType == ExpressionType.Parameter) {
sb.Append(m.Member.Name);
return m;
}
throw new NotSupportedException(string.Format(“The member ‘{0}’ is not supported“, m.Member.Name));
}
}

internal ObjectReader(DbDataReader reader) {
this.enumerator = new Enumerator(reader);
}

public IEnumerator<T> GetEnumerator() {
Enumerator e = this.enumerator;
if (e == null) {
throw new InvalidOperationException(“Cannot enumerate more than once“);
}
this.enumerator = null;
return e;
}

IEnumerator IEnumerable.GetEnumerator() {
return this.GetEnumerator();
}

class Enumerator : IEnumerator<T>, IEnumerator, IDisposable {
DbDataReader reader;
FieldInfo[] fields;
int[] fieldLookup;
T current;

internal Enumerator(DbDataReader reader) {
this.reader = reader;
this.fields = typeof(T).GetFields();
}

public T Current {
get { return this.current; }
}

object IEnumerator.Current {
get { return this.current; }
}

public bool MoveNext() {
if (this.reader.Read()) {
if (this.fieldLookup == null) {
this.InitFieldLookup();
}
T instance = new T();
for (int i = 0, n = this.fields.Length; i < n; i++) {
int index = this.fieldLookup[i];
if (index >= 0) {
FieldInfo fi = this.fields[i];
if (this.reader.IsDBNull(index)) {
fi.SetValue(instance, null);
}
else {
fi.SetValue(instance, this.reader.GetValue(index));
}
}
}
this.current = instance;
return true;
}
return false;
}

public void Reset() {
}

public void Dispose() {
this.reader.Dispose();
}

private void InitFieldLookup() {
Dictionary<string, int> map = new Dictionary<string, int>(StringComparer.InvariantCultureIgnoreCase);
for (int i = 0, n = this.reader.FieldCount; i < n; i++) {
map.Add(this.reader.GetName(i), i);
}
this.fieldLookup = new int[this.fields.Length];
for (int i = 0, n = this.fields.Length; i < n; i++) {
int index;
if (map.TryGetValue(this.fields[i].Name, out index)) {
this.fieldLookup[i] = index;
}
else {
this.fieldLookup[i] = –1;
}
}
}
}
}