分享自己做的一个简单的查询表达式模拟（ESQL,Linq)(1)

完全的模拟表达式非常困难，因为C#目前不支持自定义操作符，只支持操作符重载（希望C#能够提供操作符自定义，如果有语言支持这个，也请告知），思路其实很简单，就是建立自己的类型，并重载一些操作符，模拟这个的目的就是在实体模型下规避直接写数据库SQL语句，用类似于ESQL,Linq语法完成这个工作，提供开发一个一致的实体应用模型：

1）参数类，用于传递实际参数，最终转换成ADO.Net的命令中需要的参数：

public class Parameter
{//没有具体实现，这里只需要模拟
}

2)表达式接口：

public interface IDbExpression
{
string Expression { get; }
Dictionary<string, Parameter> Parameters {get;}
}

3)表达式基类：主要是一些操作符号重载，一些无法用表达式的地方定义类似的函数

public class DbExpression : IDbExpression
{
private StringBuilder _sqlStatement;
private  Dictionary<string, Parameter> _Parameters;
public StringBuilder SQL { get { return _sqlStatement; } }
public DbExpression()
{
_sqlStatement = new StringBuilder();
_Parameters = new Dictionary<string, Parameter>();
}
public static DbExpression operator >(DbExpression e1, DbExpression e2)
{
DbExpression theExp = new DbExpression();
theExp.SQL.AppendLine(e1.Expression + " > " + e2.Expression);
theExp.AddParams(e1);
theExp.AddParams(e2);
return theExp;
}
public static DbExpression operator <(DbExpression e1, DbExpression e2)
{
DbExpression theExp = new DbExpression();
theExp.SQL.AppendLine(e1.Expression + " < " + e2.Expression);
theExp.AddParams(e1);
theExp.AddParams(e2);
return theExp;
}
public static DbExpression operator >=(DbExpression e1, DbExpression e2)
{
DbExpression theExp = new DbExpression();
theExp.SQL.AppendLine(e1.Expression + " >= " + e2.Expression);
theExp.AddParams(e1);
theExp.AddParams(e2);
return theExp;
}
public static DbExpression operator <=(DbExpression e1, DbExpression e2)
{
DbExpression theExp = new DbExpression();
theExp.SQL.AppendLine(e1.Expression + " <= " + e2.Expression);
theExp.AddParams(e1);
theExp.AddParams(e2);
return theExp;
}
public static DbExpression operator ==(DbExpression e1, DbExpression e2)
{
DbExpression theExp = new DbExpression();
theExp.SQL.AppendLine(e1.Expression + " = " + e2.Expression);
theExp.AddParams(e1);
theExp.AddParams(e2);
return theExp;
}
public static DbExpression operator !=(DbExpression e1, DbExpression e2)
{
DbExpression theExp = new DbExpression();
theExp.SQL.AppendLine(e1.Expression + " != " + e2.Expression);
theExp.AddParams(e1);
theExp.AddParams(e2);
return theExp;
}
public static DbExpression operator &(DbExpression e1, DbExpression e2)
{
DbExpression theExp = new DbExpression();
theExp.SQL.AppendLine(e1.Expression + " and " + e2.Expression);
theExp.AddParams(e1);
theExp.AddParams(e2);
return theExp;
}
public static DbExpression operator |(DbExpression e1, DbExpression e2)
{
DbExpression theExp = new DbExpression();
theExp.SQL.AppendLine(e1.Expression + " or " + e2.Expression);
theExp.AddParams(e1);
theExp.AddParams(e2);
return theExp;
}
public DbExpression Parentheses(DbExpression e)
{
this.SQL.AppendLine("(" + e.Expression + ")");
this.AddParams(e);
return this;
}
public DbExpression Where(DbExpression e1)
{
this._sqlStatement.AppendLine(" WHERE " + e1.Expression);
this.AddParams(e1);
return this;
}
public DbExpression From(params DbExpression[] Tables)
{
this.SQL.AppendLine(" FROM ");
if (Tables != null)
{
int i = 0;
foreach (var item in Tables)
{
if (i == 0)
{
_sqlStatement.Append(" " + item.Expression);
}
else
{
_sqlStatement.Append("," + item.Expression);
}
this.AddParams(item);
}
}
return this;
}
public DbExpression Select(params DbExpression[] Fields)
{
_sqlStatement.Append(" SELECT ");

if (Fields != null)
{
int i = 0;
foreach (var item in Fields)
{
if (i == 0)
{
_sqlStatement.Append(" " + item.Expression);
}
else
{
_sqlStatement.Append("," + item.Expression);
}
this.AddParams(item);
}
}
else
{
_sqlStatement.Append(" * ");
}
return this;
}
public void AddParams(DbExpression e)
{
if (e.Parameters == null)
{
return;
}
foreach (var p in e._Parameters)
{
if(this._Parameters.ContainsKey(p.Key)==false)
{
this._Parameters.Add(p.Key, p.Value);
}
}
}
public void AddParams(IDbExpression e)
{
if (e.Parameters == null)
{
return;
}
foreach (var p in e.Parameters)
{
if (this._Parameters.ContainsKey(p.Key) == false)
{
this._Parameters.Add(p.Key, p.Value);
}
}
}
public virtual string Expression
{
get { return _sqlStatement.ToString(); }
}

public virtual Dictionary<string, Parameter> Parameters
{
get { return _Parameters; }
}
}