分享自己做的一個簡單的查詢表達式模擬（ESQL,Linq)(1)

完全的模擬表達式非常困難，因為C#目前不支持自定義操作符，只支持操作符重載（希望C#能夠提供操作符自定義，如果有語言支持這個，也請告知），思路其實很簡單，就是建立自己的類型，並重載一些操作符，模擬這個的目的就是在實體模型下規避直接寫數據庫SQL語句，用類似於ESQL,Linq語法完成這個工作，提供開發一個一致的實體應用模型：
1）參數類，用於傳遞實際參數，最終轉換成ADO.Net的命令中需要的參數：
[csharp]
public class Parameter 
    {//沒有具體實現，這裡只需要模擬 
    } 
2)表達式接口：
[csharp]
public interface IDbExpression 
    { 
        string Expression { get; } 
        Dictionary<string, Parameter> Parameters {get;} 
    } 
3)表達式基類：主要是一些操作符號重載，一些無法用表達式的地方定義類似的函數
[csharp]
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; } 
        } 
    } 
 
摘自 hawksoft